for a long time in the 17th century the Dutch army was neglected and eventually turned into militias (IIRC).
OK, but a country of 2 million people just cannot field the same amount of men like France could - unless it is totally dependent on mercennaries. So, granted, you could have a much more effective and larger Dutch army. But would it be sufficient?
The problem is that the southern Netherlands is a far more attractive part than the north west of Germany. Antwerp, Gent, Brugge, Ostend and Brussels are far richer and more interesting than whatever Munster, East Frisia, Bentheim etc can offer.
I totally agree that the Southern Netherlands are mor interesting. But I wouldn't brush of NW-Germany completely: NW Germany can provide additional manpower for the Dutch army.
The most important point, though, is that the Southern Netherlands will be the goal of French expansion, as they are interesting for France as well. If the Netherlands are united, this grants repeated warfare with France. If the Hapsburgs hold the Southern Netherlands, they will be the natural ally of the independent Netherlands, which might get additional manpower from the secure regions of NW Germany, which never experienced as much warfare as Belgium did IOTL. We should not forget that pretty much every war in Western Europe in the 17th or 18th century was fought in Belgium as well.