If Carrol thinks that "Abolish the Prison System" is a useful demand to raise, then he (or others who share his perspective) should mobilize around it.
Similarly, those who disagree should mobilize around something else instead.
I see absolutely no reason to expect that we'll all agree on what, exactly, we should mobilize around, since a decisive answer would require a crystal ball. So why debate it?
Whatever else you think of his argument, Carrol's certainly right that "[l]eft demands are NEVER simply implemented." I have absolutely no fear that Charles Manson will be freed before I return to the States.
I'll continue to work on Palestine stuff for a while. (I'm not claiming there's anything inherently "left" about national liberation struggles, although they lay necessary groundwork, but that's a discussion for another day.) Those with different priorities can work on prisons, or immigration, or any number of other things. How is that a problem worth trying to resolve?
If I can pull a Carrol, so to speak, I would opine that left debates about the best causes to mobilize around are the most useless kind of left debates. If you think a flag is worth flying, go fly it. And if you achieve any sort of success at all, the scent of victory will draw more active support from the rest of us than your most eloquent arguments ever could.
-- "Hige sceal þe heardra, heorte þe cenre, mod sceal þe mare, þe ure mægen lytlað."