There is simply no compromise with Republicans, they simply won't do it, and recent history shows it. How do these people not fucking get it? The debate is within the democratic party, and requires one part staking a position from the start that isn't in itself compromised. Would a president Bernie veto a bill that lowers the Medicare age, or expands Medicaid significantly? Or one that expands the exchanges while tightening how much the insurance companies have to spend on care and not profits or administration? He wouldn't, and I think his vote for the ACA shows this. He would continue to fight for M4A, as well he should, but barring that he wouldn't deny health insurance to millions in need because it is not perfect.
The point of the so called purity tests is what do you want to see as an endgoal? As the M4A example, do you want to see a future where there is no profit in health insurance and health insurance is guaranteed without the need for bullshit bureaucracy regardless of employment? Or one where profit or bureaucracy is largely fine, and the people left bankrupt or out of the system are tragic losses.