If history teaches anything, NOTHING is irreversible, save death.
Change immigration law to allow more Europeans in and less third world countries, get white blue collar men stoked to vote for their best interests, change the regime and crush the party and rebuild it if necessary.
That would reverse a lot of things and within years, not decades.
Considering the majority of European nations are receiving immigrants and gave birth rates below replacement levels, you aren’t going to see mass immigration from Europe anytime soon, regardless of the laws. Better to go to an Australian style points system based strictly on skills and ability to support oneself. Family reunification must end as well.