It’s not a carrot, it’s a political razor wire. You cannot charge through it, and doing so is a strategically bad move that only makes things worse (also like that trope where multiple people die in a row doing the same thing they are warned not to do). And thus far it seems like progressives in the house/senate pushes the party left (or at least slows rightward movement) more than failed 3rd-party runs ever will. Particularly because of the legislative process and actual chances of votes passing.

Also unless I’m missing something, “candidate is too progressive to win against the greater evil, so we must choose corporate” is the biggest part of the Democrats that ratchets us down here. If everybody ignored that and voted for progressive candidates, the party would be progressive (at least until establishing some form of ranked voting prevents spoiled votes).

So it’s that, grassroot movements, and/or something else. Using your half (if even that) of an ineffective system to change itself is not going to work quickly or reliably. The reason many things are so bad is shady groups on the right made long-term plans to reverse old losses, and then used their money/positions/influence to inch towards them over decades.