In your letter, you commended Sanders for working with the Democrat party, rather than running independently. I would like to highlight how that is perceived by Democrats: this is based on my personal conversations with people in the party, from aides in the House to local treasurers:
Imagine I came into your office and was like "Oh, wow, y'all are doing [software development] all wrong!" and then tried to convince you to do it my way. You don't agree, so I start badmouthing your company to the press. That doesn't work, so I start badmouthing YOU. That doesn't work, so I start threatening you - run your private company the way I, an outsider, say, or I will go invest in your competitors, THAT'LL show you.
While not true of all of his supporters, in many ways its like they showed up two days before the product release and went NO DO IT MY WAY OR NO WAY. They're willing to give the election to Trump rather than support Democrats - but want us to give them the same respect we would long-term members of the party. They're willing to harass employees - but want us to treat them as though they're simply trying to make sure we hear them.
Honestly, it seems absurd to me that anyone who still is a die-hard Berner thinks they should be treated with kindness. They've made it clear that they would rather hand the country to the GOP than vote Clinton. And I can kind of understand that - the Breaking Bad mentality of no half-measures. But it's gone beyond that. You have Berners, prominent ones, who are loudly disagreeing with Bernie about his own platform.
It's gotten to the point where even if Bernie says "I'll support Clinton if she gets the nomination," they'll still write his name in. It's childish, short-sighted, and yet comes to the table under the impression that they have just as valid a say as the people who have been at the table for years. I could keep incoherently ranting about how silly the entire thing is, but it isn't voters like you I have a problem with.
You saw an opportunity to reform the Dems, see that it might fail, and so will take your reformation efforts to the next most likely buyer. That's smart. That's political. It's respectable.
Saying that you'll kick over the table and throw the gameboard in the fire isn't.