Name a single protest taken over by an outside group. It's not something that happens. Maybe some of the white supremacy groups whose presence alone requires people to reject them or be tainted by association, but nothing on the left. That's not an actual problem on the left, much less one "on the left forever".

My first protest was organized against the Iraq War, but in practice also contained more broadly anti-Bush protesters as well as groups promoting third parties, communism, and veganism. None of those were a problem or threatened to take over the protest in any way.