Compromise is essential to democracy. Party establishments exist to enforce the agreed upon platform. If you don't like your party's platform, you can leave the party. It is harmful to the party to have people remain inside the party, but work to undermine it. Parties break down when large numbers of members start defying the platform. It is natural and even desirable for this to happen occasionally. However, it is also natural for parties to resist this from happening. If a party breaks apart, it is because the coalition of voters that sustained it could no longer agree on a platform. If a party puts down an insurrection, it is because the voters decide that the benefits of the party platform compromise is greater than the disadvanteges. In our present situation, we have the republican coalition breaking down, and the platform being rewritten before our eyes. The democratic party is also being challenged, but shows no sign of breaking down. Polls indicate that the majority of Sanders supporters, like myself, will accept and support Clinton if she wins. In the republican party, however, large numbers of people say they would support Trump if he ran as a 3rd party candidate, and other large numbers say they wouldn't vote for him, even if he is the nominee.
I used to think that the US's two party system was a bad one, but then I started following Israel's multi-party system, and found much to recommend ours. Both systems, and other systems of course, have their own advantages and disadvantages. You must choose which advantages and disadvantages you prefer.
You don't think that Bernie's supporters would follow him out of the Democratic Party were he to run third party?