Again: Grealish plays on the left or as a #10. Those are our best 2 positions. Sancho plays mostly on the right, where we have pretty much nobody (as a natural), but is also quality on the left and can play as a 10 if needed to rest Bruno. Gives us far more versatility. Sancho will be cheaper than Grealish. Sancho is 5 years younger than Grealish. Sancho doesn't have a history drunk driving away from a party and making a mess right at the beginning of lockdown. Not saying he's Rashford off the pitch, but Grealish is undoubtedly a bit of an idiot off the pitch.
As an individual, Grealish might be better if you are building a team from scratch and want the individual who can do the most on their own. But we need more players like Sancho who will make the right decisions and generally just perform at a high level with the overall team and bring the team up a level, on the right wing. That's the biggest thing. You look at our squad and rank every position in terms of need, I'd have it as:
RW
CB
DM
RB depth
CM
ST (after Cavani), so not asap
GK (sort out de gea/Henderson)
Rb starter
LB
LW
#10
8/9/10, the left back, left wing and #10 positions just don't need any attention, at all. We have our 3 best performing players in those positions on a regular basis, our 3 most important players, with Telles/Martial as depth for LB/LW and then for #10 we have Pogba for now (or Van de Beek but hes not a 10 for me). So why buy Grealish when 2 of our 3 best positions in terms of long term starting prospects play in the 2 positions he plays in? He's a quality player, but he's not our need, at all. All there is to it. Focus on filling our gaps, as there are quality players available that fit those. Not take a quality player for our strongest position and ignore the gaps. That's not how you build.