I think those players have often had to fill in for the lack of possession Dembele leaving has caused - so yes, he will be doing the role they've often had to.Eriksen and Alli have played deeper roles this season, a role someone like Ndombele could perhaps fill?
It wouldn't necessarily be Grealish for Eriksen. Sometimes perhaps Moura for Eriksen with Grealish on the bench.
It all depends on how good Grealish actually is and what his potential is. If he's not at the required level for us short or long term that's the answer, regardless of if Eriksen stays or leaves.
If he's good enough to be a positive signing if Eriksen stays he should be good enough to be a positive signing if Eriksen leaves. Though other players might have to be signed as well...
I think the question with Grealish is a difference of backup Vs first team. Is he good enough to be a squad player? Would I prefer to bring him on over Sissoko? Is he a better use of a squad place than N'Koudou? The answer to all of those is yes.
If the question is whether I'd be happy having him start in Eriksen's position then the answer is No. Noatter who else we buy.