Establishment Democrats will ensure that we get a President worse than Trump.
Literally the opposite is true. A Sanders/AOC presidency would result in an extremely reactionary GOP nominee.
A Sanders/AOC presidency that either was effective in passing some sort of radical change or seemed capable of doing so would be more likely to deliver a moderate Republican nominee, I think. The Republican donor class would go all-in if genuinely threatened and would prioritise electability. By contrast, the regular nomination of economically moderate Democrats and the compromise they champion has signalled weakness to the Republican base and it's mostly social issues that animate them anyway (so AOC wouldn't be much more likely than HRC to provoke reactionary sentiment, and Sanders much less so than either of them).
If their policies were actually unpopular, the Republican party would be able to fight elections primarily on economic issues again and win over the upscale Democratic voters who don't feel their assets are threatened by the current incarnation of the party. The more economic issues and recent (as opposed to hypothetical) policy reforms are brought to the forefront of an election, the less the culture war will dominate. If their policies were popular (presumably because they worked) - the other hypothetical we'll entertain - then the middle class would grow, polarisation would shrink and the nomination of a reactionary nominee would become less likely.
