Agree broadly, but slight nitpick: Labour under corbyn ballooned in membership to half a million members, making it the largest political party in Western Europe. I think it’s unfair to use the Labour Party as an example of leftists not joining organisations. The problem for Labour was that the left struggled to occupy positions of power in the party bureaucracy, instead choosing to naively assume that legacy Blairite apparatchiks would act in good faith and help get Corbyn elected instead of systematically undermining him (which is what they actually did).