Were an incredible band.

Why did the Red Hot Chilli Peppers suck with Dave Navarro on guitar? They shouldn't have. The RHCP were a great band as well (Mother's Milk and Blood Sugar are amazing albums). Doesn't make sense. It does not make sense.

So, on the flip side, good choice band replacements that worked:
Jane's Addiction: Eric Avery with Flea (for reunion tour)
Van Halen: David Lee Roth to Sammy Hagar (and then Gary Cherone? oh dear)
Metallica: Cliff Burton to Jason Newsted (even though I still think he's a tool)
Ozzy: Randy Rhoads to Jake E. Lee (questionable) to Zakk Wylde (awesome choice)
Iron Maiden: that first singer with Bruce Dickinson
Dream Theater: Kevin Moore to Derek Sherinian (questionable) to Jordan Rudess (awesome choice). They go through keyboard players like they're going out of style
Anthrax: Joey Belladonna with Charlie Bush (completely different styles, worked well)
Alice In Chains: Mike Starr with Mike Inez

But by far and away, the best effort I would say:

Faith No More: Chuck Mosely with Mike Patton. There's not a shit show in hell that FNM could've gotten that big without big Mike screaming his little insane lungs out.