He is, but is he willing to sit on the bench like they are? He could be starting for someone else
He would not be a benchwarmer. Neither Rashford or Martial are consistent enough to consistently keep a player like him on the bench and Greenwood is still young. Add that to the fact that Bruno and Pogba may need rest or can get injuries and you have an ample amount of games that he will play, particularly with cup games and UCl fixtures. People also need to get into the mindset that competition for places can exist. If he proves to us that he is a better player than Martial, Rashford or Greenwood, he would replace them as our starters. If he is not, he wouldn't. I don't see why that is such a problem.
Furthermore, City have Bernardo Silva, Gundogan and Gabriel Jesus on their bench. Would you think they would be willing to sit on their bench when they signed? Do you think Grealish thinks Rashford, Martial or Greenwood are better than him?
At Real Madrid, players like Isco, Asensio and Jovic sit on the bench, at Bayern Goretzka, Javi Martinez, Tolisso and Sule sit on the bench. At Barcelona, Vidal, Umtiti, Rakitic and Dembele sit on the bench. I don't see why its such an issue.