I think its a good deal for United but that fans need to have reasonable expectations.
De Ligt is a weird player because he was so precocious that he acquired a reputation as a player who must have a very high ceiling but it turned out that he was just a good all around CB without elite attributes, a guy who developed early, is perfectly good enough to play for a big club, but doesn't have the athletic traits or ball playing skills that would make him among the elite.
He'll absolutely improve on United's current options and I don't see that much chance that the club will look back on the deal as some kind of huge mistake in a few years. But equally there's very little reason to believe that you're buying the Toni Kroos of defenders. If he was that level player then Tuchel would not have tried to replace him last summer and handed his position to Kim to start last year, Bayern would not be selling this summer, and he'd be getting into the Dutch side ahead of players like De Vrij.