"Communism" ala USSR, North Korea, etc just moved the problem to a strictly worse area, and something you could not get away with. At least in capitalism, there are better and worse 'masters'.
If the "modern socialist" types gained tons of traction tomorrow and became mainstream within the democratic party people like Feinstein and other authoritarians won't stop pushing regressive authoritarian policy. They'd just do it in a way that's sculpted to line up with the (newly) popular "socialist" sentiment. So the end result would be pretty darn Stalinist IMO.
So yes, you're right that current socialists are generally not in favor of secret police and shipping undesirables to the gulag but you're also wrong because if socialism becomes popular it will include all the totalitarians on that side of the isle who do want secret police and gulags.
As regrettable as it may be I don't see any path for going from the status quo to something more socialist without the current crop of totalitarian jerks in the ruling class turning it into a 1984 dystopia (the technical, procedural and much of the precedent for said dystopia already exists) along the way.
I don't believe that currently ostracized strains/ideas/people will somehow inherently come to power just because democratic socialist ideas become mainstream. Democratic minded socialist are just as inoculated against authoritarianism as liberals. These people were persecuted and killed by ML/Stalinist authoritarians too.
I hope that "anyone who believes in capitalism is therefore right wing" is not a mainstream European perspective. You can have a pretty darn socialist government without seizing the means of production.
>I don't believe that currently ostracized strains/ideas/people will somehow inherently come to power just because democratic socialist ideas become mainstream.
The fact of the matter is that these totalitarian beliefs are not nearly ostracized enough for that statement to be anywhere close to reasonable.
On one side of the isle it's considered perfectly reasonable to violate all sorts of rights as a hurdle to buying the tools of self defense. On the other side of the isle it's the same thing but for abortion. This is but one example. These kinds of people won't just find a way to use the popular movement of the day to push their beliefs.
Sure they might make things more socialist if that's what the people demand but don't think they won't also work on their totalitarian pet project at the same time.