They had no choice. Apparently they made L4D2 to secure the IP after Turtle Rock split off.
TLS did a bunch of dumb shit nobody asked for. Some of it was based off a list of demands Valve made if they wanted to release the update in the first place. Updates don't always equal 'good' and what it added was superficial at most. There is an insane amount of custom campaigns out there of better quality than the 2 part campaign of The Last Stand. Just go to workshop and play a few yourself. Adding the CSS weapons officially - that were only accessible in unofficial servers or campaigns prior - was a stupid idea. Many of them aren't as solid as the mainstay of L4D2 weapons and it's always irritating when a bolt action AWP is in the place of what could have been the M16, AK47, or the semi-automatic sniper rifle. Something like the CSS knife feels out of place and the two new official weapons added are wonky at best with their hit detection. Screwing up the voice radial was extremely petty for VS and neutering it in coop was even dumber. Including many of the cut voicelines (Valve cut them in development for a reason) feel off or not in tone with the official lines.
But my biggest issue was how it broke a lot of custom plugins/campaigns. You're talking about a game that hasn't had a major update in about 8 years, so the community nestled in on the fact that the game will be consistently stable without any future incidents that might break mods. Every 4-6 weeks, a group I'm in plays a slew of custom campaigns we find on workshop/gamemaps on Expert on Saturday night. It's modified in a way that the more players in the game (we have around 16 at peak), the more special infected spawn, the larger the Tanks HP is, and the bigger the horde count is. Makes shit challenging regardless of player count. Plus everyone is given a medkit at spawnroom, and few other fun plugins like Character Select Menu (think Hank Hill, Moon Man, Carl from ATHF ect).
Well the update and its minor 'patches' since then fucked it up. The recent surprise big 'patch' for TLS (
https://left4dead.fandom.com/wiki/Updates_(Left_4_Dead_2)#June_15.2C_2021_.282.2.1.3.29) caused most of the plugins to fail, again. We had to postpone our l4d2 event by a week, again. Thankfully, we are grateful we have somebody who knows how to code and patch up plugins (some are still not functioning properly), but that can't be said for other communities utilizing these old ass plugins that are no longer supported by the original authors. It's a pain in the ass if nothing else. Ultimately this is the core audience who still plays L4D2 to this day, not the people who jump on for a weekend after not playing it for 6-10 years.