This is true for a lot of countries. The huge electoral break of the European Far Right in the last few decades is at least partially due to voters who always were there returning to their natural habitat, which they had left when that habitat became too toxic after '45. (I'd say)
I must say I seriously doubt that. I think it might be true in some countries on the continent (like your Belgium or say Austria) but not in Scandinavia. For one thing, the WWII far-right was a lot more right-ish than the current far-right.