There's no gauge change. Spain high-speed tracks already use the international one (older tracks still use Iberian gauge). There are already high-speed trains that go to France without stopping at the border.
Right, but that's only if you stick to hi-speed rail. Which isn't a reasonable limitation if you want to travel to Spain more broadly (or, for that matter, from the Netherlands more broadly!)
The point isn't only that there aren't perfectly reasonable routes - there are, and that's great! It's that the overall network is pretty fragmented, and that that does cause issues. If you live in Amsterdam and want to travel to Paris - you're in luck. If you live in Murcia and want to travel to Groningen, you're out of luck. And if you live in the countryside... well, don't even bother.
The point isn't only that there aren't perfectly reasonable routes - there are, and that's great! It's that the overall network is pretty fragmented, and that that does cause issues. If you live in Amsterdam and want to travel to Paris - you're in luck. If you live in Murcia and want to travel to Groningen, you're out of luck. And if you live in the countryside... well, don't even bother.