On the other hand, there is no challenge left for him at Ajax, except doing well in the CL, which is extremely hard and he anyway already reached the semis once. Otherwise, he can only repeat what he's done or fail. Plus the lower end of the Eredivisie features clubs that would hopelessly relegate from the 2. BL or English Championship, and stadiums that would be tiny in the 3. BL. (But I see there are also stadiums below 10k in the English League One!)
Also, if Ten Hag really wants to make his way up internationally, it might make sense for him to go to a subtop club in a top league, to prove himself in a different and more competitive environment. I mean, while dominating the Eredivisie with Ajax certainly isn't a given, it might be a more impressive accomplishment, and more noticeable to the European top clubs, to achieve a strong season with a club like Leverkusen or Spurs - even if that would only result in CL qualification and a decent cup runs.
Not that this is the better perspective; I'm just trying to say that, after some years at Ajax, I'd get it if Ten Hag would think like this. (Him renewing his contract might indicate that he's more of your mind; but he might also be doing like their players and guaranteeing Ajax a nice transfer fee in case of his departure.)