When you have a chance to go for a super talent, you go for the super talent. You worry about filling out the rest in time as those kind of players are much more easily attainable by virtue of them not being quite as big of talents.
Secondly, you would rather your rivals get those "multiple options" you guys have listed rather than get a super talent like Sancho who could be a thorn in United's side for 10 years or so.
We have internal options for other positions worth assessing over for at least another season. For midfield we have Mejbri, Garner, and Levitt. For RB/LB we have Laird, and Williams. For CB we have Tuanzebe and Mengi.
No, that's not to say they will make it. I'm not delusional. The point is to say it is possible for them to make a mark and be a squad option if given the chance. It's a game of numbers and the more talents you have, the more likely one talent makes it. For example, if Garner goes out on loan and does really well, that's 1 problem solved. Same with Mengi.
If Mejbri is a super talent and takes really well to first team football, that's one squad option filled. Don't count on it, but it is a possibility. On the other hand, a player like Chong is someone I would not take that gamble on.
Point is, it's worth waiting a year to assess our situations for those positions if it means we're able to acquire a super talent like Sancho. Those kinds of players don't come around often. Much less, an English prodigy right after their age 19 season and just about to go into their age 20 season. It's not like we're talking about a Zaha type talent.