Why? IIRC before his assassination there were plans to give Willem van Oranje those kind of titles.
I think I see this differently. The power of the Stadholders didn't decline if they were weak. It declined if they were absent. Willem IV and V were not very strong stadholders (to put it mildly) yet they became the most powerful stadholders in history! In your timeline we have Willem II surviving after he historically died at the age of 24. Let's say he survives to the age of 50. That's 26 years he can use to solidify his hold on the country. He can also transfer that power to his heir Willem III and thus the Estates don't have a Stadholderless Era to regain ground lost. If we look at history it has never happened that the Estates gained power against a sitting Stadholder. I do think that we could get a situation as in Great Britain however
Giving this scenario more thought I think a good reasons for the Netherlands to ally with France could very well be a prolonged Parliamentarian England given the Oranje-Stuart connection![]()
With a prolonged life of William II undoublty there will be more children, who could be intermary with French nobles, and why not an daughter of Orange with the Dauphin.
All this would greatly change the balance of power in Europe ( as far as this concept was recognized at that time) The European power strugle would be even more focussed between Habsburg and Bourbone with the Dutch at the side of the Bourbone.
In OTL wars were fought in OTL Belgium, would in this time line the diplomatic differences between the two major West-European factions be battled out in the German lands? And at a consequence if the Dutch Republic is heade by a Arsitocratic family with monarchist ambitions, would it expand to the East at the expence of German Principalities and domesticly at the expence of whealth, welfare and maritime trade and maritime power?