Couldn't Griezmann work as a midfielder as well? He seems to be quite solid defensively. I'm sure he wouldn't want to do that, but it could be interesting. He is very versatile.
What If they know Suarez is leaving after next season? Then it would make a lot of sense, because you're not going to find anybody as good as Griez for 100mil€.
Midfielder? No, for sure not, he could play as a right winger or second striker and that's it. Only Coutinho can play as AM out of those 4. (Suarez, Dembele, Griezmann, Coutinho)
It makes sense If Suarez/Messi are willing to rotate more and are fine taking easier games off. I don't think Dembele seems type of player to sit on bench and I don't think we can afford that either. Dembele cost us too much to be just benchwarmer.
There is no way Dembele is going to be a benchwarmer you don't have to worry about that. Bare in mind that Demebele has on 20 years so the best from him is yet to come. He will hit his peak probably way after Suarez retires.
I also sincerely doubt that Suarez will be our starter in the 2019/20 season so only next year will be the problematic one. So my opinion is that we are preparing for the life after Suarez with this Griezmann move.
Also "rotation" means that we would rest one of them in every game. If we consider that Messi will play majority of games and Coutinho will play in Iniesta place than you have 3 players rotating for 2 positions ( Griezmann, Suarez and Demebele). That would mean that one of them is rested in every third game or so. We'll see when and if he comes, but it could be interesting.
To be honest in some home games against some weaker opponents all 4 of them could play together with Messi in 4-3-3 or 4-1-4-1 formation. I see no problem in playing this formation against Alaves, Leganes, Las Palmas, Levante or Malaga at home.
------------Busi------------
---------------Coutinho----
-------Messi----------------
Griezmann--------Dembele
------------Suarez-----------
And than in 2019/20 4-3-3
---------------Busi--------------
---MIDFIELDER-----Coutinho---
Griezmann---Messi---Dembele
or 4-4-2
Dembele--Busi--MIDFIELDER---Coutinho
------------Messi----Griezmann-----------