Tommy Kaverick replacing Roy Khan in Kamelot. The band hasn't been quite the same since. Although, to be fair to Tommy Kaverick, nobody else could have adequately replaced Roy Khan either.

When Michael McDonald took over for ailing Tom Johnston, the Doobie Brothers became a different band, and I personally didn't care for their sound after that. But others did, and so it basically comes down to taste.

Bands inevitably morph and reinvent themselves, and more often than not, make more fans when they add new members, or pivot in their style. It's difficult to imagine Journey without Steve Perry, Fleetwood Mac without Christie McVie and Stevie Nicks, or Dream Theater without James Labrie. Some hardcore fans of the early Genesis sound regretted Peter Gabriel's departure, but drummer Phil Collins was a diamond in the rough. Speaking of talented drummers, I wonder what it was like for Dave Grohl to be overshadowed for as long as he was.

Another Van Halen mistake. Replacing Michael Anthony with Wolfgang Van Halen.

Paul Rodgers in Queen. Love them both but together it was like chutney and ice cream

So when David Lee Roth left to make his movie (which never was made) and start a solo career, who would you have replaced him with? Like Ronnie Montrose after Sammy Hagar left the band that bore his name, Eddie and Alex didn't think Van Halen's run was finished (disagreeing with their record label), and their first album with Sammy, 5150, was their biggest seller. Now I have "I can't drive 55" as an ear worm ... thanks for reminding me of Sammy!

Gary Charone was a worse choice than Hagar.
Another flash in the pan (if he even produced a flash) was Blaze Bailey, who briefly replaced Bruce Dickinson in Iron Maiden. Nice to have Dickinson back!