Widespread misconceptions of what capitalism and socialism/communism actually are.
Socialism - i.e., the control of the means of production by the working class - is no more "the other side of the coin of capitalism" than the abolition of slavery is to slavery. Socialism is not about people giving up individuality for the entire society, or bringing everything under state control, etc. It is about the producers of all value in society - the working class - ending its exploitation (the extraction of surplus value, which is the source of profit) at the hands of the ruling class. Calling for socialism is no more extreme than Nat Turner calling and acting for the abolition of slavery. We're talking about classes here, the exploited and the exploiter.
These "mixed systems" you speak of are capitalism. I'm sure you're thinking of Sweden, etc. as "mixed systems" - but where in Sweden do the workers control the means of production? Either private or state capitalists still extract surplus value from the working class in all of these places.

Actually unlike communism – which fits your description of the working class owning the means of production a lot better – socialism is a system in which public enterprises/cooperatives controls the means of production. And if capitalism is a system in which a country's trade and industry is controlled by private owners for profit then they sort of are on polar opposites of the spectrum. Unless I'm reading something wrong.
My point was simply that these polar opposites don't necessarily translate well into reality when discussed as which is better/worse on a permanent timeline. You mention Sweden and state capitalists extracting surplus value not being socialist. I'm simply saying that if so called "state capitalists" extract surplus value (taxes) to use them on social programs (not-for-profit) that's not exactly capitalism either.
Karl Marx himself considered the capitalist class to be the most revolutionary in history due to its abilities to improve the means of production (through competition). But, much like I'm trynna do, he also argued capitalism was prone to periodic crises i.e. when capitalists would invest more in tech than in their workers (decreasing the salaries to dangerous levels) i.e. the slavery you see in the US today.
We know that Marx believed that pure communism would emerge after the collapse of capitalism and the "dictatorship of the proletariat" – which is why he details the rules of labor so well in Das Kapital I presume – but I don't think he ever put much of his intellectual thought into the conditions for communism or even socialism to work (yes I know they are quite different) on a permanent timeline.
I haven't got much against the three systems, even if I prefer capitalism better due to its revolutionary qualities. What I do have is a gripe with people that try to pit these three systems together to decide which is better for an economy overall, as I believe that to be economically ignorant. Because in my view, the economically mature thought should be: "How to juggle these three systems so that a country may be able to quickly adapt to each and every one of them at a periodic crisis?"

unless you're a social or cultural conservative, not economically