> Mainstream parties could take a hard stance on immigration and destroy „far right movements“ overnight.

No they would not. It would just empower far right to make further demands as everything shifted toward them. And you can even see it practically, each time mainstream parties move toward right, far right becomes stronger. Meanwhile, anti-far-right voters end up without anyone to vote for.

Becoming far right yourself does not cure far right, it makes far right stronger. Far right voters wont vote for you, why would they? And voting for you achieves nothing, you wont oppose far right anyway.

> Hmmm, maybe it is this ignorance destroying the democracy?

No, it is far right who is openly trying to destroy democracy. And helping them wont save the democracy. Blaming mainstream or left for what far-right does also does not help democracy.