While I personally think most champions could do with some improvements, I feel the general sentiment is that players don't want a champion to be reworked, especially their own favorite champion, unless it is absolutely necessary, and even then, it should try to stay as close to the original as possible. In their push to update the champion roster, Riot has often run into the issue of reworking champions purely for the sake of a rework, or because they just decided from the start that the champion would be an easy target for their roster update. When that happens, the rework is often done with a very limited scope, and ends up being not as pristine as it's expected to be (and often it highlights that champion's need for a larger rework), which ends up frustrating a ton of players because the rework felt unnecessary. On top of this, you sometimes get pretty obvious knee-jerk reworks like Janna's recent changes, or Kassadin's rework right after he had a 90% ban rate, which tend to be extremely superficial, yet somehow still damaging to the champion's gameplay. Effectively, at least some part of the community feels like Riot has been a bit too trigger-happy in the past when it came to drastically changing the gameplay of certain champions, including in ways that didn't need to happen, which is why they're less willing to see champion updates in the future.
To be fair, though, we as players view the game through a very different lens from Riot, and often our own opinion is biased because of a champion or particular playstyle we love, even if that is the very thing that is unhealthy. For example, a small contingent of players were upset at the Sion rework, not so much because of anything in his new kit, but because they genuinely enjoyed playing the old AP Sion with the point-and-click stun + nuke and massively damaging shield, which was one of the least interactive playstyles in the game. The same thing happened with Poppy's rework, when a few players disliked the removal of Diplomatic Immunity, arguably one of the worst-designed abilities in the game. Most players tend to be protective of the champion they love (which is understandable, and a good thing for the most part), but some tend to take that to a different degree and categorically refuse anything but pure buffs to their champion no matter what, which you can sometimes see on Gameplay when Ahri gets brought up. Effectively, while Riot has not always been reasonable in how they've reworked champions in the past, many players aren't really reasonable with how they expect Riot to balance or design their game, so in many cases it's a matter of looking at the context and the reasons being put forth for changing a champion, or leaving them in their current state.