Griezmann is not a center forward so i don't see how he can be Suarez replacement, i think he would just occupy the same spaces as the players we already have, unless he wants to play on the wing again
Dembele is a much better winger than Griezmann, Coutinho is also a singificantly better wide player than him.
Griezmann isn't a natural striker or a winger, his best role is as a CAM/SS; so basically the same position or area Messi plays in right now. I'd imagine he could do a job as a striker and the board are bringing him into to gradually phase out Suarez in a year or two.
There is no way we'll be able to play with three attackers clogging up the centre of the pitch (Griez, Leo, Luis). Even if Valverde tries something like that it will miserably fail, hell we're already having some problems with just two players; Messi and Suarez playing centrally.
He'll be seeing a lot of the bench here for a few years until Suarez eventually leaves.