I don't really see what the problem is here?
The moderate/establishment type Republicans are without a party at the moment. If you want change in this country you need to build consensus.
Sure, you can win with just Democrats. Quite comfortably, even. Obama did just that in 2012 - independent voters went for Romney. That's clearly done jack-all for his agenda. Even big-ticket items like immigration reform, which passed the Senate with a wide majority floundered in the House, which was too busy shutting down the government because the president wouldn't agree to repeal his signature legislative achievement, the one that's literally been named after him.
Voters - specifically, Obama supporters - had an opportunity to punish them in the 2014 elections. They rewarded Republicans for their obstructionism by staying home. It's okay, there was probably something really good on Netflix or something. I mean, I can't tell the difference between a governor and a Congressperson, can you? Exactly.
Snark/shaming the people who form a large contingency of Bernie's current base aside, the point is you can win a majority with just Democrats. In our current political climate, you need a hell of a lot more than that to get anything done.
The more reasonable Bernie supporters act like all he needs as president is a Congress with 218 Democrats to pass single-payer and free college, break up the banks and give everyone a unicorn. Obama had 256 Democrats when he passed the Affordable Care Act. Do you know how many votes Democrats scraped together? 219. Same for cap & trade (which ultimately didn't pass the Senate). Dodd-Frank fared slightly better with 223 votes in its initial vote and 237 when a few loopholes were opened to draw Republican support in the Senate. Oh yes, the Senate. There is that pesky 60-vote requirement. Democrats could do away with it, but it's a useful tool for the minority, so I doubt they will. I don't necessarily agree, I'm just saying. Senate elections are also staggered so it's going to be a while before you've filled up the chamber with 100 Bernie clones.
Now, that's for the Bernie supporters who acknowledge the reality that is Congress. For the ones who think we can just get 5 million college kids to lobby out the windows of Paul Ryan and Mitch McConnell, or swamp the hotlines of the Supreme Court to lobby the justices (who by the way are completely insulated from public approval by constitutional design)? I really have nothing to say to you, as it will fall on deaf ears. Good luck with your twitter revolution.
There is no tyranny of the majority. The presidency is not a dictatorship. If you think Hillary is a corrupt shill and Bernie is a liberal savior and that will make all the difference between what they would actually accomplish once in office, let me put it to you this way - half the country thinks like Trump. Some won't want to admit it, but the reason Trump is doing terribly in the polls is all style and no substance. You've got some pretty severe opposition there, and a stupid and uninformed opposition, which is the most dangerous kind. If you think Obama has been a disappointing president and attribute inaction on many important legislative proposals to him "not trying hard enough" at best or just being a corporate tool at worst, that is a fundamental misunderstanding of the American government that the Republicans have become quite adept at capitalizing on.
If there are Republicans with reservations about Trump who are thinking about switching, they should be welcomed to the fold. If they donate money to and vote for Democratic candidates, they are doing a great deal more for the liberal movement than any keyboard warrior ranting about how Bernie can totally win 70% of the vote in California and turn the primary on its head. That's even if they vote straight ticket Republican for everything else, because strong Hillary coattails will produce a durable Senate majority (possibly holding off a loss in 2018 due to inevitable midterm drop-off), and possibly even flip the House.
Even with a Democratic Congress, Hillary won't be perfect - she'll likely come up short in some areas, and be downright shitty in others. But you know something? Equal pay for women, equality for the LGBT community, fighting for criminal justice reform and reducing systemic racism in this country, expanding on Obamacare and Dodd-Frank, passing comprehensive immigration reform, increasing the minimum wage, curbing climate change, increasing and improving alternative energy, free 2-year college and pre-K, reforming the student loan system, operating the government with some basic level of competency and returning to pre-sequester (prequester if you will) spending levels, raising taxes on the wealthy, building a nationwide high-speed rail, protecting net neutrality, protecting abortion access, working towards cures for cancer and Alzheimer's and better research and treatment for autism, implementing a system wage insurance, protecting Social Security, Medicare and Medicaid, reauthorizing the Voting Rights Act, restoring McCain-Feingold, flipping the Supreme Court majority - if Hillary can do even half of that it will absolutely be worthwhile and meaningful. And presidential candidates historically keep 75% of their promises, so there's plenty to look forward to.
If Republican donors want to be a part of that, be my guest.
tl;dr - just stop