They could have utilized a lot of what they developed as ways to supplement the core Halo game. Imagine an aliens style gametype where that beeping mod was on and you have slow movement whole flood are super fast and have camo etc.
They developed all these redundant weapons, instead of having them all in play, why not things like Elite Slayer or Forerunner Slayer where instead of UNSC weapons, you start with a carbine and plasma nades or light rifle and pulse grenades and all the weapons on the map are the faction ones only.
Look how much fun action sack was for a while. Imagine if the game wasn't built around what 343 wanted to be, but that it was supplemented with these other gametypes. I'm sure all of us wouldn't have minded playing a few rounds of infinity Slayer in social skirmish after a few matches of ranked team Slayer etc.
But no, classic decade long Halo became the niche and this random clusterfest of loadouts, perks, power weapons, ordnance, sprint, armor abilities became the mainstream Halo.