Signing Neymar for obscene amount of money is surely an end to Mbappe hopes, when he decides to move on.
Because, first, if Neymar leaves, a big reason is Mbappe. He won't want him at Barca, and would throw tantrums like he usually does. Mbappe would also not want tonplay second fiddle to Neymar, when he will hit his actual peak.
Second, we wouldn't be able to compete with RM financially if a bidding war happens.
Atm, its better to keep the squad same and try a new manager if things don't work out. If it still is in jeopardy, go ahead and get Neymar, who will be older & thus much cheaper, plus, there might be some new talents available aswell.
I wouldn't mind if he moves to RM and costs them 200M. I would rather focus on getting De ligt and De jong atm.