This line of thinking completely ignores the fact that policy is hard, working solutions are complicated, and the best answers come when the most perspectives are considered and debated in good faith.
It is not immoral to question the effectiveness or realism of a policy, particularly when in order to even come close to enacting anything like that policy, you need to win with the support of a lot of people who have (often well-founded and thoughtful) doubts. The electability argument is absolutely a moral argument if the ability to actually achieve moral reforms depends on being elected.
That’s not to say Democrats should water down their ideals, or be timid in their defense of them. These ideals are what makes them the party that deserves to govern America. But their solutions still have to make sense and have a chance of winning broad support before they become anything more than fantasies.
If the Democrats nominate a candidate who endorses everything on the progressive activists’ wishlist, but then loses in November by scaring the majority of Americans who find those proposals too extreme, your ideological purity contest will have achieved nothing and stolen America’s only chance of defeating Trump.