Yes, Bagladesh is screwed, but if that [Clathrate gun](https://en.wikipedia.org/wiki/Clathrate_gun_hypothesis) goes off, we're looking at ~60m of sea level rise. The Low Countries aren't going to do very well at that point, either.
So even if 60m of sea level rise happened - and most of the Netherlands could not be saved - I still think some portions would be OK. Amersfoort, Arnhem, Apeldoorn, Nijmegen and Enschede (as well of Maastricht of course) would be saveable with creative water engineering. Combined, you could probably fit several million people there, easily enough to maintain the Netherlands as its own country (again, this is apocalypse-level planning here.)
Additionally, the rest of the displaced millions are mostly very highly educated, wealthy, English-speaking people who are allowed to live and work anywhere in the EU. Compare this to Bangladesh.
Additionally, the rest of the displaced millions are mostly very highly educated, wealthy, English-speaking people who are allowed to live and work anywhere in the EU. Compare this to Bangladesh.