"Punching left" just means antagonizing Socialists.
"Socialist" is an incredibly broad label. To argue that critique on auth-left groups is an attack on socialists is just not meaningful in any way, as it specifically refers to a niche within socialism.
Secondly, systems do not allow themselves to be changed. Feudalism wasn't voted away, nor is Capitalism.
Except that historically speaking they have changed in certain situations. They are rare of course, but it's certainly not unheard of. The Second Hellenic Republic for example was established via democratic referendum, after which the monarch was peacefully deposed. The idea that all opposition is somehow controlled is fairly ridiculous given an honest reading of many historical events.
There is of course a certain set of safeguards built into almost any system that resists changes. A constitution is a good example. But that too can in most systems be changed. Resistance to change doesn't mean resistance becomes impossible. Authoritarian governments tend to establish blocks that prevent change, sure. But most democracies would be able to for example remove capitalism if a sufficient majority votes to do so.
There is a level of conflating of ideology and political system that you seem to display, which I suspect is somewhatideologically motivated in your case. Then again, those distinctions are hardly ever really truly clear. One could argue that capitalism is a strictly economic ideology, not a political one. But any system that adopts it also sees effects in the political sphere.
I don't think the case that viewpoints like fascism should be allowed makes any sense
Never argued fascism should be allowed. It's an ideology that is a clear and present danger to society and human life, so it should in my opinion be banned.
Decentralization doesn't mean democratization, such analysis would mean Capitalism is more democratic.
Capitalism by definition centralizes capital as much as possible. This accumulation of wealth leads to an accumulation of power, which has anti-democratic effects (see: the US). Decentralization does not necessarily mean democratization, but centralization does almost always lead to more authoritarianism.
It's the big stumbling block of communism as well. It attempts to decentralize wealth by spreading it over the population, with the workers owning the means of production. But doing so requires incredible power (to seize and redistribute), which typically ends up with a small group or even a single person. And they rarely relinquish that power (see: the totalitarian leadership of the USSR), which also leads to authoritarianism.
As for Marxism-Leninism, you can oppose it without drawing equivalence to the Nazis.
I was very explicit in that I don't consider them equal. You can compare things without equivocating them. To consider a comparison an equivocation is what people do to silence critique, a tactic which I don't appreciate. All I'm saying is that both are beyond the boundary of what I in good consciousness can support. I don't care about counting skulls, I care about the risk that the pile grows. That risk is far greater with Nazism (obligatory: fuck Nazis), but unfortunately also not insignificant enough with Marxism-Leninism either.
Thankfully those aren't the only two options available.