I understand that's the logic going into this. But those are massive shoes for anyone to fill, especially a 20 year old.
I mean we have had absolute shit to show for big money transfers - Sanchez, DiMaria are the obvious names but even Pogba has at best middled, and the jury's still out on our skipper.
A player costing 120m, 250k+ a week in wages and all this fanfare and expectation...i'm a little wary after all these years. He'd have to put in 7/10 performances pretty much every game he starts with very frequent 9 and 10 performances. That's not a small ask at Man United.
Whilst he might have to put in those sort of performances to not be criticised by the media, that doesn't mean he won't have been of value. Again 12m per year would be a bargain.
There are so many factors to those purchases you mentioned. For a start, Sanchez wasn't particularly expensive, his wages were but the transfer wasn't. He was also much later in his career. Di Maria was also much further in his career and ultimately didn't want to be in England. None of those things apply to Sancho.
Pogba, in my opinion, was a great purchase. He is unfairly criticised because of his price tag but I'd rather have him in our team than not and under our current manager he seems a different player, particularly mentally. Shirt sales, media attention & ticket sales also arguably covered the fee.
Sancho will be amongst friends and compatriots. He is a young player than hasn't even established himself in the England squad really and has everything to do to prove himself, unlike Sanchez and Di Maria.
Sancho will be battling another potential 'generational talent' in Mason Greenwood.
Have you noticed the energy and support between players like Martial, Rashford, Bruno, Pogba & Greenwood. Sancho will feed into all that.
United are on the up and everything surrounding the club is positive, except a few transfer pessimists, that wasn't the case when Di Maria, Sanchez or even Pogba joined(despite cautious optimism at the time).
That would be my defence of the transfer fee. I think it will be money well spent for a prolific young English player, playing for Englands biggest club.