Well there are multiple things that would classify as 'too much damage'. You're right, a Rengar killing a squishy target in 0.00 seconds is too much damage, but an adc killing someone in 4 autos from 600 range whilst also shreding through tanks would also fall under that description too.
Bruiser meta maybe, but a lot of the top tier fighters have been nerfed significantly from where they were a couple of months ago, but assassin? Not really. Assassins have always been a particularly popular class, but are too feast or famine to ever have a meta evolve around them.
I am a Bard otp, I'm not and never expecting to one shot anyone, so please don't make such assumptions.
An enchanter correctly utilising their kit doesn't just negate burst, their shields and heals had insanely low cooldowns to make them great against all sorts of playstyles. Bruisers either lacked the damage to kill them or lacked the tankiness to survive against them. Poke mages had their abilities absorbed by a shield and life steal. Tanks had little to know damage to pose a significant threat. All this power from abilities that only require you to point and click on an ally. That does not provide a significant window of counterplay like you somehow seem to believe exists.
It's fair because enchanters have been too powerful since their inception. All enchanters sat at around a 52-54% win rate every patch (occasionally higher) for years, which is insane. The ardent meta accentuated all of these problems and it was obvious that they needed to be brought down significantly to be fair.
Enchanters didn't get compensation buffs because they didn't need them. With the new support items they are now thoroughly a late game class and if you reach that stage of the game you are innately more powerful than a Thresh or Leona etc. There is absolutely no reason for a late game class to be powerful early game as well.