Why would a Sanders supporter want to risk putting someone like Trump in office (through inaction)? Asides from maybe the anti-establishment aspect, Trump and his proposed policies spits in the face of everything Sanders stands for. Even if we ignore his own dangerous, egomaniac tendencies, having a Republican president in office with a supreme court nomination on the line could easily undo all the progress (however little) the US has made in areas like healthcare, marriage equality, minority rights, gun regulation, climate change, etc, and very possibly even regress on certain areas.
There's so much at stake in this election and a Trump presidency will be absolutely disastrous for not just the US, but the entire world.
Resigning oneself to naked fascism with the idea that it could somehow lead to more progress down the line is quite the gamble. Perhaps it could work out, but you're playing with some serious fire.
It's anyone's guess how Trump would actually rule, but throwing dissenters in jail would be admitting defeat on his only real core value, which is that he's so great that he always gets his way on charisma and "negotiation skill".
Other than that, I do agree that "it must get worse before it can get better" is bad policy, since the former is usually a lot more reliable than the latter.
That's a pretty ignorant attitude.
The next President will be appointing 2-3 SCOTUS judges and a slew of Federal judges. You're totally cool with letting Trump pick those people?