Grealish is a good player in a position where England have an abundance of good players. Attacking midfielders and inside forwards? England have those. What they don't have is a midfielder who'll link the play between defence, midfield and attack. Rice and Phillips sit quite deep so they can protect the defence and link defence and midfield, but the gap ahead of them is massive. That allows a team like Italy to control midfield and essentially take the English forwards out the game. Grealish isnt the guy to change that. He doesnt work well enough as a proper 3rd midfielder. I'm not sure that Mount does either. Henderson is really just another deep lying midfielder option and they already have those in Rice and Phillips.
England need midfielders. I think Sterling is a weak link because he doesnt track back to support his midfield. If you're playing with 2 central midfielders then that leaves them liable to being overwhelmed by numbers and unable to do much to win the ball back. There would always be an passing option for the opposition to use. Bellingham may well be the answer to that problem and they could play him as part of a midfield 3 - similar to Rangers playing a midfield 3 and Italy going with 3 in the middle of the park with support for their fullbacks.