Any rational capitalist (or more realistically, mixed market supporter) should agree that other systems are theoretically possible, and should probably even support small scale scientific tests of whatever people want to realistically propose.

This already happens with tests of UBI occurring all over and examples of coops existing in many places as well. UBI is too new to say but looks promising, and coops seem great in certain areas of the economy if properly supported but not optimal everywhere, as far as I'm aware.

However, if someone thinks their system can only work with absolutely everyone in society participating after a revolution where the sinners (whoever they are) are eliminated, they really ought to recalibrate their beliefs or join a militia if they're really serious.

Can you point to a socialist country where it has resulted in better outcomes than its peers? Cuba might be a contender but then there's also Venezuela next door...

I do not consider China to be a socialist country. It is a market economy where your average Foxconn employee no more controls the means of production than your average Detroit autoworker. My understanding is that China doctrine states socialism is one big long term TODO (with ever moving goalposts), requiring their economy and material wealth to have grown first. Well, you can't deny it's grown but I'm still hearing a lot about Chinese billionaires while there's also a huge swath of Chinese rural poor.

I can agree with the "so far" qualifier. Things can always be improved, and yes many other systems look great on paper. I am just waiting for the evidence of those other systems functioning in practice. Personally, I would not my country to be the guinea pig in case it backfires. I am content with my western country and its generally current standard of living over past generations and civilizations.