> seemingly extreme proposals serve to stretch the Overton window before the middle-ground wins out
I've not seen any evidence that this is true, nor that "Overton"-thinking has any evidence as such.
OP's comment is precisely that backreactions to this kind of thinking are often larger than the alleged shifts that are assumed to occur.
NB. The "overton window" is a description of a consensus, not a model; ie., there is nothing of the consensus which is like a dial to be moved.
The reason that people view X as possible is the latent preferences around X, incentives and institutional practices, and their (essentially rational) judgement about the practicalities and possibilities of X.
Eg., Marxism isn't "outside the overton window" by some weird dialogue exclusion. Ironically, it is only in the conversation because of "overton thinking" ie., hype.
It is outside the window because peoples preferences, incentives, and institutional practices are overwhelmingly oriented away from Marxism; which has no practical or evidentiary basis. No social mechanism of implementation. No group of people have the incentives, reasons and power to implement Marxism.
Activism cannot "move the overton window". It provides no reasons, incentives, or power. It changes no institutional practices; it offers no evidence.
"Hype activism" frequently does more harm than good, by reinforcing the status quo.