Sales dont lie.
Marvel are in a two year sales slide that doesnt look like ending anytime soon, and whilst I'm certain that there are many things that have contributed to that, it would be foolish to think that the treatment of their characters and their ongoing PC-crusade havent played its part, or that the unhappy voices that are easy to find on any discussion forum or comic book shop you care to choose are simple people who 'dont read comics' and can be casually dismissed.
As for who theyve 'ruined'
Captain America - Nazi
Ironman - 'dead'/evil AI
Thor - unworthy
Logan - 'Dead'
Nick Fury - Now the man on the moon
Hulk - Dead
Pym - evil robot
The list goes on
Pretty much every classic character has been killed, disgraced, or otherwise marginalised - and I have no doubt thats a quite deliberate tactic by Marvel, because theres no way that their new characters can suceed whilst the originals are still centre stage.
Its a fact though that even having shuffled-off the classic characters, the new characters have STILL failed to spark the audiences imagination.
Compare the sales of pretty much any title you like with their all-new, diverse replacements and you will see that the replacements heroes to a greater or lesser degree perform more poorly to their classic counterparts.
The likes of Thor has on average only sold a few thousand less books than the previous Odinson-centric Thor title did, but others like Ironman (Riri Williams) have performed very poorly in comparison to the early Stark-centric volume.
The only replacement hero who has actually sold more comics than the hero they replaced has been Ms Marvel, which continually outsells Danvers book, although it is not a top selling book in its own right either.
In ongoing serial fiction, characters get killed off (And replaced.) and resurrected all the time. Heck, do you think that there wasn't a backlash when Gwen Stacy died? Cap being a nazi, that wasn't the real Cap, was It? Superheroes go evil all the time, and later on that almost always gets retconned. Remember Hal Jordan? Or The Crossing? Etc. In these cases, ruined means don't like the stories or directions.