If you're looking for a serious interpretation of what actually happened to the trajectory of the (mostly liberal) Left in the last few decades, consider this article by George Hoare (part 2 here). Some quotes follow:

The Left populist experiment is now very much over. From Bernie Sanders’s failure to obtain the Democratic nomination for President in 2020 to the destruction of Corbynism, from Jean-Luc Mélenchon’s unsuccessful bid for the French presidency to the capitulation of Syriza, the waning of Podemos, and the eclipse of Die Linke by Alternative für Deutschland, Left populist movements that once held promise have now been decisively defeated across Europe and beyond. Left populism typically presented itself as an alliance of young, educated, and energetic activists on the one hand and older working-class voters alienated from neoliberalized, establishment parties on the other.

Despite the revitalized party (at least in numerical terms), Corbynism did not prove successful at the ballot box nationally. There are a number of different ways to read the failure of Corbynism to translate from a movement within Labour to a ruling political force. The first is that the movement collapsed under the weight of its internal contradictions, unable to maintain the alliance between young, urban, and university-educated progressives at the core of the new membership and of Momentum, and the ordinary working people catalyzed behind the new leadership.

A more critical reading goes further: the project was actually a class project of the lower half of the progressive managerial class, who ventriloquized the working class (and in some readings despised them)

[...]

At the same time, the activist base for Left populism—the “lower PMC” of university-educated urbanites finding only insecure employment and unable to afford housing in the metropolitan centers where the high status and “interesting” or socially valuable jobs they seek are overwhelmingly clustered—will not disappear.