Every band member of a long running and established band says that the next lineup change is the death of the band, only to find out that when it really happens, they can't let go of their life. At a certain point is a way of life, a habit, what you actually are able to do, and it's not easy to let go of that. I'm sure if you'd ask John Petrucci if Dream Theater is done without Myung he'd probably say yes, but I'd "dare" him to face the absolute and certain fact that Myung (or Rudess) wants to leave right now, today, and have him face the harsh reality of Dream Theater being dead. I'm sure he would not be ready to lay it all to rest.
RE: Marko being "replaced", well, I stand by my statement, but when you put it in terms of "they found a bassist, figured out what to do with the vocal parts and moved on" yeah, I can only agree with you.
About Nightwish being done without Floor... of course they would technically survive, people would still show up at concerts, but unless they find the best replacement ever and put out the strongest album ever, I could see them downsize in concerts attendance and importance on the bill of festivals. They could still have a carrer, but it would be the last phase of their carrer and they would go out relatively quietly and not headlining sold out shows and being the biggest name on festivals. Well, let's hope anyway to never find out if I'm right or wrong.