What we do with gametypes isn't a landscape issue, it's a resource one. Most folks are heads down on the next title, so moving people over requires asking the question each time about the trade-off in how soon we release the next title vs. adding to the current one. We sometimes choose the former, sometimes the latter.
As to the audience overall, that's a harder question. How a game ships definitely affects its audience, but once you have a given audience in your game, built around your game's playlists and modes, you have to consider that audience from then forward. So, for Halo 5 at least, things have changed in the comp audience.
We have a lot more of that audience playing in comp playlists than we've had in previous titles. In past Halos, the comp audience that actually played the comp playlist (MLG) was pretty small, and didn't have much of an impact on other playlists. In H5, a lot of them actually play the comp playlists that we have now, so changes we make in either have a much stronger effect than they used to. Whether this was caused by how we shipped, or by the overall growing size of eSports across all games, I don't know. But we have to plan for it now.
As to whether shipping H3 style would work in a modern title, I also don't know for sure. The thing is, when H3 shipped, it was the only major shooter until around late 2009, so we had plenty of audience and it was safe to run lots of modes between Ranked and Social, because running new modes back then was kind of like releasing a new game.
When we shipped Reach and 4, though, both with somewhat similar approaches (though evolving) it didn't work the same anymore because the shooter landscape is now super saturated. If you want a certain different gamemode, it's actually more likely you'll find a BETTER version of it in a game that's fully dedicated to that mode, rather than finding it in any given game (like Halo or another shooter).
Successful games today tend to put more focus into a smaller amount of differentiated modes. So you're more likely to go to another game than look for another playlist.
Because of that, most games just don't have the population to spread their players around too much --- even games that have much bigger populations than Halo has ever had (I've worked on a few of these) still try and focus the population.
I could see a world where we improve the custom games end of things so players can have what they want easier though, but balanced against the mainstream matchmaking.