Gramsci described this process - “the long march through the institutions”- as an inevitable mechanism rather than a deliberate policy. It does pretty much amount to the same thing.
Most of these guys imagined they could keep their subversion secret.
Fabianism, et al ~ the whole Socialist steaming pile ~ is just another way of thinking about theft, and these guys all thought they'd invented a new and secret way to do it.
Rule #1 about theft ~ there really aren't any new ways.
Rule #2 about secrecy ~ there really aren't any secrets ~ just stuff you don't know about yet.
Eventually they get uncovered, we know who they are, and they can be punished ~ which is why we need to keep the death penalty handy for what seem to many to be petty offenses.