For United, it makes a lot of sense. Ole talked about how United used to have a lot of quality options in attack. When he was at the club, Ferguson went ahead and signed Yorke for big money when he had already 3 quality forwards. Ole would also want more than one quality option on the bench. If he signs Sancho and Grealish, that will solve this problem in a big way. Not only Grealish could slot in for Fernandes and Pogba but also Rashford and maybe Sancho if he has to.
For Grealish, it's all about how much he believes he could displace one of our first XI. Personally, I don't think there's much between him and Fernandes. Grealish is exactly the type of character who would think the same and will be up for the challenge. Also, being an important squad member in United is worth more than being the captain of Aston Villa.