Depending on the game, it can be reversed, but it's not easy. Diablo 3 is a recent example. Bad launch with major server problems and gameplay issues. The 2.0 patch and expansion basically undid all of that, got rave reviews, and AFAIK did result in a sales bump. That's Blizzard, though.
Assassins Creed usually sells a lot up front, sells some DLC, then moves on. DLC doesn't provide a "reset" to try and win people back over the same way a major expansion launch does. For AC Unity, the damage is done. Fixing it makes sense to try and limit the damage to the brand and because some of those fixes can probably be used in the next game, not because they hope it'll generate more sales.