Nowhere near his prime - true. Already "so good" is more debateable. He hasn't really stood out when playing for England so far. And playing in the Bundesliga is less challenging than playing in the Prem.
If Son is better (and I'd say he clearly is), and if Son is not worth £108m (which he isn't, by a long chalk), then it seems to me that your £108m valuation argument for Sancho is based mostly on his young age and your belief that (a) he'd adapt well to the Prem; and (b) will improve to the point where he becomes not just better than Son, but much better than Son.
I would have thought that United could far better spend £108m in other ways than on such a risky punt.
Sancho being 20 is a big part of his appeal. If you knew he would be as good as prime Son for the next 8-9 years, he would definitely be worth £108m in my opinion.
I think the biggest thing with Sancho that is rarely discussed, but I'm sure United has the analytics people in place to know about, is that he has really really really massively outperformed his xG and xA numbers the last two years, especially this past year. Some of that may be high quality finishing but even the best finishers don't outperform xG by much in the long run. And nobody should really outperform xA much in the long run as its dependent on how other players finish.
Sancho had 17 goals and 16 assists in the Bundesliga on an xG of 9.3 and an xA of 9. By xG+xA per 90, he was a very good player but not really a standout player in the Bundesliga - pretty similar to guys like Breel Embolo or Christopher Nkunku (actually behind both of them) that nobody is lining up to spend over 100m for. That's what I would be worried about if I was thinking about spunking £108m on him. One of the things United should be asking themselves is "Would we pay this much for the player if he had 11 goals and 9 assists last year, rather than 17 and 16?" Maybe the answer is still yes, in which case you go for the deal. But if the answer is no, then its probably not a good idea.