Eredivisie isn't really that competitive compared to other European leagues we have, you'll always see Ajax, PSV, Feyenoord, and AZ on the top four since there is really no competition going, that's why these teams can sell their top players all the time since they know what they have is enough to maintain their spots in the league.
The eredivisie can indeed be a less competitive league, but with Ajax and Ziyech perfomances in the champions league, i can say he is a quality addition to the chelsea squad.
For Ziyech moving to Chelsea I don't know why he agreed to it in the first place since I don't think he will be one of the main guys for the team considering that they already have a lot of good players in the position he main plays. If he don't do anything to become the real deal for the club his playing time and role would diminish quickly and he won't be that top prospect that other teams will be targeting.
With Ziyech being a wide forward, Pedro Ageing, Willian Uncertain, Barkley average. it is the right step by chelsea and Lampard