If I'm understanding you correctly, your argument is that Halo (the first one) is the last game where individual skill was more important than the ability to work as a team. (I'm not suggesting that the ability to be a team player isn't a skill - I'm trying to summarize your overall point, that Halo allowed individual skill to shine and gave greater depth to team play than later games did.)
While I use Halo 1 as my example game, I don't think it was perfect and there are certainly some changes that could improve it. That said, on to your post...
I don't think you can really compare the two skills like that. Their importance varied depending on the situation; the environment of Halo CE (local LANs primarily) meant that any group of players usually had vastly disparate skill sets. In those types of situations, not only did you not see much teamwork, there was usually one or two individuals who steamrolled everyone else just through blunt individual skill. This made up the bulk of people's experience with Halo 1.
Teamwork became vastly more important when you actually got two teams of roughly equivalent skill or at least players who were familiar with what they could or couldn't do. The depth of viable strategy was greatly increased by the weapon and power spawning syste, the player respawn system and the utility weapon. Simply put, there were more viable options in any given situation that could be pulled off through a combination of individual ability and teamwork.
You also began to see players find success by being really good either through individual ability or their ability to work off their teammates. In later Halo games, you will only find success through your teammates.
The part I don't get, then, is why the early competitive winners were so quickly eclipsed when Halo 2 came out.
If you look at the early days of competitive gaming (first national championship was in 2002, AGP was pretty active in 2003, MLG kicked off that year), you'll see names like HP Darkman, StrangePurple, Zyos. They dominated at the start (especially when they teamed up to form The Dream Team) - but soon after Halo 2 came out, they ALL faded away. It's like they couldn't adapt to the new game (though the Ogres and Walshy, who'd all been around in 2003, had no trouble doing so).
I can't speak for all of the old-schoold elite players, but I am still friends with one of the players from that first National Championship (which didnt' really feature the best at the time, but the best they could find) you're referring to. He stopped playing Halo because it was a completely different game that he didn't enjoy, not one he couldn't be good at.
The term "adapt" is thrown around a lot. The irony is that the reason a lot of players left was because it was too easy to adapt, there wasn't as much "adapting" required, and the skill gap was narrowed.
So if a group of guys could dominate the competitive scene, both individually and as a team, when the game was DEEPER, and rewarded individual play more... why wouldn't they dominate even MORE in a game that became less deep? (Zyos kept playing FFAs through 2006, and then dropped out of professional gaming, but he stopped playing team games in 2005, and the rest of the Dream Team was gone in 2004.)
Because the differential between them and teams below them got narrower. A shallower skill gap benefits lower-skilled players much more than it does higher skilled players. Especially when you reduce something like the shooting skill, which everything else hinges on. Players who used to have a large advantage because they had great dexterity and could react and think quickly suddenly saw all those advantages nullified to a great extent.
The difference between Halo CE and Halo 2 was massive. The way Halo was played changed forever, and a lot of the great CE players simply didn't enjoy the changes.
Part of the answer might be as simple as "the competitive scene wasn't big enough to support them and they went back to school" - the first real contracts weren't signed until mid-2006. But I don't think that's all of it. To me, as an outsider (to the competitive scene), it seemed that players that were BETTER individually (in Halo) had more trouble adapting to the team dynamics of Halo 2 - it's not that Halo's team play is deeper, it's that Halo's team dynamics are LESS important than the individual skills of the members of the team. (That is: four great FFA players could easily dominate in Halo team play - but 4 great Halo 2 FFA players wouldn't necessarily dominate in Halo 2 team play.)
I guess, for me, it boils down to "Team playing is a different skill than solo playing, and its importance was greatly diminished in the first Halo, compared to later games."
I agree that teamplay and individual play are different skillsets, to a point; there is some obvious overlap in the skillsets. "Teamwork" is not something that you need to force in a team-based game, like Halo. It's something that will always be present, and its depth will grow when you allow for different playstyles to find success.
Tying this back to your original argument, it seems we agree... to a point. Individual skill is MORE important in Halo than in Halo 2. Where we seem to diverge is that you are suggesting that higher individual skill will necessarily lead to higher team skill... and I'm suggesting that there's not really a correlation (at least in the first game). I will ABSOLUTELY agree that in Halo 2 and 3, teams with greater-skilled players dominated, and I will agree that in the early days of Reach, wins were more unpredictable because there WAS a greater 'random' factor - one that MLG removed over time, leading to fewer upsets and more predictable outcomes. But what I don't see is a greater 'depth' in Halo gameplay that comes from individual skill - what I see is that individual skill level was far more important than the ability to work together as a team. To me... that's LESS depth.
Wow, I didn't expect this to be that long. And now I'm late for a meeting.
My original point was that a game that allows an individual to be a deadly, effective force has more potential for deep teamwork and strategy than one that doesn't. The reason being is that it allows players to seperate themselves from their teammates, cover more of the map and attack from unexpected angles and have a realistic shot of success.
However, basic strategy like Teamshot is going to be much more prevalent in a game that restricts the individual and certainly much more prolific. My question would be; is that better? Is it better to have the outcome of the game rely so much on the most primitive form of teamwork, especially when its due to the fact that its literally forced on players?
Look at how team strategy has evolved. It started in Halo 1 as an incredibly dynamic thing, where teams would prioritze powerups and choose how to allocate their resources and move around maps. Individual players would each be working on a different objective and playing off their teammates.
Halo 2 moved to a static-setup from of strategy, where an initial coordinated rush was followed by attempting to hold a ceratin power position. Individual players who struck out on their own would find they couldn't realisitically take on more than one enemy at a time, and quickly found that sticking to one area with teammates was the best way to succeed.
Halo 3, team strategy resembled Halo 2's, but more like a swarm of zerglings, where success primarily came from having more players shooting at the same thing.
All the strategies of Halo 2 and Halo 3 were perfectly viable in Halo 1 as well, but there were also so many more strategies that could work. It only appears that teamwork was less important because it was less in-your face. It's easy to look at a team all sticking together and say, "that's teamwork." In reality, I see greater depth in a team that has a couple of players sticking together directly attacking enemy positions, another across the map contesting a power weapon and another sneaking in for a flank.
Decreasing the importance individual ability and saying it increases teamwork is like saying increasing the size of the basketball hoop will increase teamwork.