Yes, this makes sense. Bitcoin is probably not suitable for replacing every and each fiat coin in the world right now as it is and make an unique, universal coin... So another coin will probably replace it, or Bitcoin will evolve so much that it will make no more sense to call it Bitcoin. Only future will tell.
yes. I think the key is development. if bitcoin stops growing I'm sure someday there will be another product similar to bitcoin that has many advantages compared to bitcoin, so bitcoin will be replaced.