First off, nobody takes Marxism by itself. If it's accepted, it's always with extra things attached and other parts removed.

What on Earth do you mean? What do people remove?

Second, my issue in this case is the Lennist part. A vanguard party degrades into cult-like behavior, and this is very consistent with ML groups big and small.

What do you mean "cult-like behavior?" Why do you believe this "cult-like behavior" arises? Where does Lenin deviate from Marx?

Why do you advocate for Syndicalism, because it sounds good to you? You aren't making any analysis nor points, you just seem to be contrarion. Surely you have some reason for wanting syndicalism, no?

I think you’re confusing two different tendencies.

Small parties that do not have a social base can become insular. If they do not realize that their limitations are set by external factors they can turn inwards and become cult like. You see that a lot in Maoist groups in western countries where there is no peasant class their brand of politics can mobilize.

On the other hand successful socialist parties that come to take power end up having to defend it from various reactionary forces, both external and internal. Being put on the defensive causes these parties to seek resiliency through centralized decision making.

That said, I don’t think either of these tendencies are unique to Marxist parties. Also, Marxism is meant to be scientific so learning from past experiences is key to avoiding the mistakes of other Marxists.