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.