The problem is that you are seeing Grealish as a CM. He's like an iniesta or a David Silva. Those players would be slaughtered as part of a two man midfield or as 8's in a 3. We would be sacrificing too much defensively, and may not even have the physicality needed to dominate midfield as you would imagine. So what we are really talking about here is Grealish and Bruno rotating for one spot which really doesn't make sense considering how many other holes we have in the team. I would understand playing Grealish on the left and pushing Rashford to the right or having him play like Eriksen under Pochettino, but not this. This would be us shelling money for the next shiny new toy. We are rebuilding, we can't afford to waste transfers.
We dont play a 2 man midfield though. We haven't in years. It's a 3 man midfield literally every game. And Bruno can play the #8 role perfectly fine. His goal threat is more worth to push him up, but he can do both to a very high level. Grealish is more a dribbler than a passer but hes also not strictly a #8 or #10, and kind of in between both just like Bruno. Those 2, partnered in front of a holding mid, is more than fine and makes perfect sense IMO. We can do that, we can push one of them around to fill in the attack and rotate Fred or Mctominay in midfield, we wouldnt be reliant on one player for creativity (Bruno Fernandes right now). People think midfielders are way too inflexible these days. They can play different roles, especially when you look at the attributes that Bruno especially or Grealish to a lesser degree have. Not at all a shiny new toy, more a piece that greatly improves the squad and gives us something different that we wont have with Pogba gone. It's just on the manager to make it work, but personnel wise, a midfield group of Bruno, Grealish, Fred, McTominay and Matic is perfectly balanced between work rate, physicality, defensive solidity, creativity, goalscoring, playmaking, dribbling and passing.
Add Sancho to the mix and we'd play:
Rashford Martial sancho
Grealish McTominay Bruno
Grealish Martial/Rash/Green Sancho
Fred McTominay Bruno
Rashford Martial Greenwood
Fred Matic Grealish
Etc...Basically a lot of options when injuries come. You need a squad. Our biggest problem is arguably not having quality depth and competition for places.
Look at Liverpool's depth. Fabinho, Henderson, Keita, Chamberlain, Wijnaldum, Milner, Lallana
City: Rodri, Fernandinho, Gundogan, KdB, David Silva, B. Silva, Foden
Chelsea: Kante, Jorginho, Kovacic, RLC, Barkley, Mount, adding Ziyech to probably be #10
Spurs: Ndombele, Lo Celso, Gedson, Winks, Alli, Sissoko, Dier
Our options are: Bruno, Pogba (leaving), Fred, McTominay, Matic, Pereira, Mata.
The latter 2 are trash, while with Pogba here it's perfect but as hes leaving, we need to replace that creativity and quality. Grealish pushes up back up to having quality starting level and depth.