You need to view things on a systemic level instead of personal participation in isolated events. If all the people who want good things will unconditionally support lesser-evilism, why would the Democrats ever give them concessions? Why would they do anything other than what they have been doing, pandering to moderates and conservatives, while the people who have some idea of what good policy would be get browbeaten into supporting them anyway?

Perhaps Trump does get elected because of this, but then that is the Democrats choosing letting Trump win over even a somewhat moderate position on Palestine, or healthcare, or whatever. What better case could you make to the people that there needs to be a new major party if not that? What better case could you make even to the fucking Dems that concessions are necessary if not that?

If you step back, it becomes obvious to the point of truism that the strategy you endorse is precisely why it's always a matter of the "lesser evil" and never a matter of any real "good".