343 did force the update on everyone originally, but after the outcry and doing community voting by offering updated and classic on the same map for comparison of their popularity they kept classic around. Good thing too, because most people didn't like the update. Super Slayer, the TU Team Slayer Playlist, always had a lower player count than regular, unupdated Team Slayer. Just looking at current stats Classic Playlists have a higher player count that TU ones, and while classic Team Slayer got a population Super Slayer is completely dead.Team Slayer is popular by default. You can't compare its population to newly introduced playlists, because players automatically gravitate towards the familiar.
The people who play Reach to this day are of course the diehard fans of the game, so it's understandable that they might like vanilla Reach and be opposed to change. However, the opinions of this niche minority aren't necessarily representative of the preferences of the larger Halo community. You'd have to have lived under a rock to not know that bloom is not a very well-liked mechanic at all.
Sure Team Slayer might have an advantage as a default playlist, but that ignores the fact that it *was* updated, the player base cried out, and then there was a direct face of between TU and classic in game. And then after 343 saw that TU wasn't as popular as the vocal proponents of change made it out to be made the decision to split classic and TU. And then, back when Reach was still popular, Team Slayer significantly outperformed TU slayer in the playercounts. It wasn't as if 343 said why don't you try the update to see if you like it, they dialed it back after backlash.
The way things played out back in the day shows that the people wanting TU were in the minority. Otherwise all of Reach would have just gotten the TU and that would be the end of the story. It doesn't surprise me that the people who complain moved away, even after the changes they advocated got implemented, because they're the ones that can't be made happy in the first place. I don't see why the game needs to be catered to the people who complain the loudest, as they are the ones who will move on the fastest anyway.
And yes, TU addressed many of the complaints people had, such as bloom, shield bleed through, and armor lock. Of those I can only really agree with the last one, as bloom allows the player to choose how accurate they want their weapon to be as a mechanic. Take the Needle Rifle in Classic, if you rapid fire more than 4 shots it becomes much more inaccurate punishing spam at long range while allowing for a risk calculation about the fire rate at medium range. In TU the bloom completely resets allowing for pinpoint rapid fire ruining the experience. If TU were to be applied to Invasion that would severely impact balance, since NR could be used to completely suppress the spawns.
Shield bleed through is an ok change, it makes Reach more like the older Halo's, but having to decide whether to risk a charge to melee/run/engage normally was an interesting factor even if there are a lot of people just charging for a double melee.
Nerfing Armor Lock was the best change in the TU IMO, but even before that there were strategies to punish it's use. The most annoying thing is it just grinding the gameplay to a halt which never really got fixed in any case.
Just to reiterate, while the views expressed in the last paragraph are just my personal opinions, the fact remains that back when Reach was popular and the TU was implemented the TU was very unpopular with the people who played the game.