I still think you're underselling De Ligt at Juventus (he needed the injuries to re-establish himself as a starter, but then he did remain one and was important), but anyway. I agree that the Ajax academy is overrated a little - if not as much as you say. I mean, they do have a great production line there, more than many top clubs (even if PSV and Feyenoord are doing fairly alright in that sense right now as well), but obviously it's for a very specific style. It's a bit like some Dortmund players recently: they stand out in their home system, but may not necessarily have the attributes to perform at the same level in a different context. I think people tend to forget that for Ajax, where the club's overall play style is applied across the academy from the youngest stages. So, transfer an Ajax player to Union Berlin or a Sean Dyche team, and there's a good chance of failure, cause they've never been trained in that style. But put them in a functional similar setup, and you have a much higher chance of success.
That's why I wouldn't necessarily think Arsenal are buying a dud in Timber, since he'll feel right at home in their setup. But I agree, it totally remains to be seen if he can make the step up to the speed and physicality of the EPL, and he is either way unlikely to become any kind of leader figure for them (unlike Martinez). So there is a significant risk that Arsenal are just buying a versatile back-up player, in which case he's pretty expensive. But given his stylistic fit, the risk shouldn't be huge (especially with the sort of cash EPL clubs have to splash around), and he might very well come good. I guess we'll see!
Btw, shouldn't you be a limited sympathetic to the Timber family given his twin brother Quinten's season at your club?