One tactic in stopping a moving Overton window is to circumvent it by introducing a compromise policy just as a position is moving from acceptable to popular, which I suspect is happening a fair amount in the quest for 'small target' on some hairy issues (immigration is always a popular one among the knuckle-draggers).

It is, as you imply, very important that the left start with building radical alternatives which are sufficiently viable that they can move from the fringe, through acceptability, popularity then policy.

I must confess I have never given this much thought but in hindsight most of my political activism operates on this basis.