The concessions obtained through SocDem goverments in European countries came about because all those countries had thousands of actual communists in varying posts in the government and the concessions were to reduce class consciousness and weaken the communists.
To understand the magnitude of difference, imagine if the US House of Reps (total seats 435) had 40 honest-to-god Communists in power, with another 80 left-of-Bernie SocDems : thats what the French legislature was like in the 60s and 70s.
Instead the US has 3 extremely mild SocDems who may or may not be grifters.
I hope you see the magnitude of difference here, there is no real way of moving the admin left and whatever minor leftward shift you may see is what the ruling class themselves, by their own accord, decide is necessary to keep the threat of civil unrest from getting too high.
So whenever you see any liberals IRL or online talk about how we need to become like Norway or whatever, remind them No Communists in govt = No European style Social Democracy
DSA explicitly says in their non-party documents, since they're the whipping dog radlib wing of the democrats and not an actual independent political party, that they will not cooperate with any Marxist group.
These leftist parties have always felt weird to me. They all invariably end up either impotent or tear themselves apart into obscurity. Im not saying they are useless, but they are just oh-so-easy to manipulate and wreck. All the effort you put in them could be gone at any time.
Thats because a part of it is because our ideology exists on the fringes of our society, and that attracts cranks due to it's position relative to societal norms, and the other part is because the Socialist movement is quite literally having to rebuild itself from scratch due to the destruction of the USSR, and the complete degeneration of the Western "new" Left into Social Fascism - primarily due to bougeoise Interference in their drive to undermine Socialist movements in their spheres.
What modern socialist and communist parties must do to become successful is in not just reading and adapting past tactics and organizational strategies utilized by past successful worker parties, but also work on expanding the parties beyond merely chipping away at the center from the fringes to making major in-roads into the mainstream of cultural consciousness as a new and distinct political power separate from the capitalist party.
This is what parties like PCUSA, or PSL are working towards, and so far they've been doing a lot better than those that came before.
I agree with the rest of your post, but I'm going to need a citation for the last bit.
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