Pogba probably not since it's also not really Grealish's position. But Rashford and Bruno? Bruno has had an incredible start to his time here but it's still only early days. I agree that as each game goes on it looks more and more likely that he's going to remain at a top level (I believe he was consistent at Sporting as well), but for all we know he might have a month or so period each season where he struggles compared to his normal standard. Or he might take a while to get going after an injury. And we've seen first hand that Rashford can be quite inconsistent. Obviously he's young and that will hopefully improve, but it would be good to have that option.
The key thing is I don't think Grealish is that far behind them. It's like having Ole and Sheringham behind Yorke and Cole. The latter two may have been first choice but the other two were close enough to keep them on their toes, and also step in regularly and start themselves. That's how I see Grealish fitting in with us and why I would really like to see him join. In theory I would say that a couple of other positions could be a higher priority for the starting 11 if we can find the right player, but in Grealish I just see a fantastic opportunity that I think we'd regret missing out on over the next 5 or more years.