Phrase for when there is no point in speaking because you'll be met with excuses or rebuttal

person-2: (disengaging) "This is < missing phrase >"

This is futile ?

Futile - adjective - having no purpose because there is no chance of success

Source - Oxford Learner's Dictionaries

The phenomenon I believe you're describing is referred to as being obstinate:

Obstinate - adjective - 1: stubbornly adhering to an opinion, purpose, or course in spite of reason, arguments, or persuasion

Source - Merriam-Webster

Jaded resignation

As in, "I used to explain the outcomes of alternative potential policies in the context of attempting to persuade others' toward my political views, but now I bite my lip in jaded resignation."