Its a great example of how word of mouth based on certain criteria can lead to collective and unanimous agreement about someone, or something. The criteria in this case being that Grealish plays for Aston Villa, so he is seen as (1) potentially attainable by every big club while also (2) not having any association with any of them, so he is pure and also free from the tribalism that leads a percentage of fans to talk down Kane or Mount or whoever else simply because they can't bring themselves to have an unbiased opinion of them.
Grealish is obviously a very talented player but he isn't a goal and assist machine to the point that he's going to be one of the greats. Listen to the average English fan though and he is the key to us either winning or not winning the Euros (if he could play as a disciplined number 8 he perhaps could be, but that's not who he is). I would still have him in my England team, just, but I don't see a huge difference in what he brings to the table compared to what Mount or Foden, or even Lingard when he's on his game, could do in that position.