Here is a slightly complicated but interesting Excel table I made comparing various Western countries and how strong the left and right have been over time.
The three columns on the left compare the strength of the mainstream left and mainstream right (excluded parties are on the far right) during three periods of European history, the “30 Glorious Years” from the end of WWII until the mid-70s, the period from the mid-70s to 2008, and the period since 2008.
The three columns on the right show the average strength of a theoretical coalition between the centrist and left-wing parties, again excluding parties not considered in the mainstream.
As you can see, the trend is mostly consistent; the left has gradually been weakening since the 50s and 60s. It’s unclear to me if this is because of cultural issues (I don’t think Europe was really grappling with cultural issues in the 1980s, for example) or simply because people felt the left had “done its job” and built a strong welfare state, so now there was no imperative to keep electing them.
The trend definitely accelerated after 2008, and part of that is likely cultural issues.
Anyway I found this interesting and I hope you do too.