By using your logic, the Democratic Party should go 3rd party.
Think that is going to happen?
Your plan will elect Hillary (like it did Bill, twice).
Well you’re in good company believing that. The entire left wing of Free Republic supports your position.
The actual on the ground reality and history is that by supporting leftist republicans, not so left republicans accomplished two things. First, they, by definition, moved the party leftward. Secondly, From personal experience, experience BTW that anyone can have by speaking with non Republicans, they are trusted less than Democrats. The mere name GOP/Republican now makes non political junkies cringe. They WILL NOT vote for one.
Now, since we have REPEATEDLY lost by NOT doing what I and many, many others have suggested over they years, perhaps it’s time to DO what people like me suggest. Because you cannot lose at any worse level than is happening right now.