I can see that with 4 travelers, it's not just the American fascination with freedom and the open road ... but a car is a burden in medieval cities. First, you need to decided on your desired destinations, THEN you will decide if a car or the train is better. (Are you prepared to leave all your belongings in a retractable soft-cover trunk all day, repeatedly?) As previously noted, boating from Koblenz to Trier is a horrible idea. Even if the train were uncomfortable (which it isn't), it's so much faster that it wouldn't matter.
For example, if you care about the Netherlands Delta Works (which we didn't have time to see), or Ypres, or Hooge Veelue, or D-Day Beaches, then maybe you need a car. But to see Haarlem, Leiden, Den Haag, Antwerp, Gent, Bruges, Leuven, Brussels, Mechelen, Lier, a car is a pain in the neck. We take a nice hotel and day-trip. BTW, what's the point of automotive freedom if one of the travelers (making this up) has a bad knee, and you have to take the awful tourist tram in Trier to think you saw the town? There are great public transit busses to take you to the slightly out-of-the-way sights, like the Amphitheater, that you CANNOT see with a rental car.
I can tell you from personal experience that it's annoying to take the train from Belgium to Lille, Flandres (Chunnel terminal.) But there are a few good connections from Antwerp, a multi-day worthy city.
Did you come up with WW I because everyone's going this year? I would make sure that everyone in the group really wants to poke around dusty museums on a nice day. I'll admit I haven't been to Ypres, but an empty battle meadow is just an empty meadow to me.