I spent a year or so on a predominantly left wing forum as one of a handful of conservatives (this was a supposedly unbiased forum for discussion). They don't care about courtesy, they don't care if you concede a point. It's a 24/7 hate train, and they will actually become more aggressive if you agree that they're right about anything, ever.

All they see is a sign of weakness to be exploited, and if anything, they're disgusted by the idea that you agreed with them.

At the very least, this principle of being conciliatory with anonymous left wingers is no good online. Maybe in person.