As an Ajacied I can tell you that in my view this is the right move at the right time.
He has won the title mutiple times at Ajax, been captain, best player of the Eredivisie, played a European final and is loved by most of the fans. He deserves to make a nice move to a top-league and Everton seems like a good match. Koeman will know him well and wont bench him very quickly, which is probably a big reason for him to choose Everton. A top 3 club would have been too big a step I think, as he isnt a specialist and isnt the most physically imposing guy out there.
However he does excell at being very allround, decent and has a good footballbrain, fantastic mentality and team-spirit. He could play every position in midfield and would allways get a 7 at least. The very critical and sceptical might find him boring or average, but they arent looking at the full picture. He is the kind of player that looks average and does ridiculously effective things that you dont expect, almost like a german haha. As oppposed to a player who looks sensational but always seems to dissapoint and is busy with other things in head, like a Depay for example.
At Ajax we are very spoiled with technically gifted players, I wont have to name names, because everybody who follows football a bit knows them. He did get some citicism because of that, but he proved himself to be a great player for Ajax and most people who criticized him had to admit they underestimated him.
I think he is a safe choice to make if you know what you are doing. He also often scores the decisive goals, or the equaliser, not the 4-0. In that respect he was our most important player of the last couple years for sure! I also love how sometimes he seems to be everywhere at the right moment, whether its defensive or attacking.
People who compare him to other Eredivisie-players, Dont! Ajax is in a league of their own when it comes to scouting, developing and maturing talent and every player that left for a big sum has turned out to be great! Maybe except for Babel and 1 or 2 I cant think of right now.
I expect him to succeed, unless he gets ridiculous injuries ( which he isnt prone to) or if the coach leaves and some dumb cnut comes in.
ill keep checking in on him, I hope you guys will be nice for him the first few months, give him some confidence, then he cant really fail.