Presumably, people who support whatever "pure socialism" is think that the USSR or Cuba didn't do it right. Which justifiably raises No-True-Scotsman objections.

But I think the argument on that point is close enough that your pithy one-liner is not responsive to the general point:

Capitalism vs socialism debates generally happen without acknowledging that the vast majority of the world uses a relatively narrow spectrum of hybrid systems. Some free markets, some social safety nets and regulations.

This is pretty fair, but there's a vast spectrum of flavors of socialist economies that have never really been executed, or only existed very briefly. Soviet-style bureaucratic central planning is a very limited slice of the possible permutations.

But the fact that the other flavors (say, anarcho-syndicalism) have never existed and seem unlikely to happen anytime soon, DOES speak volumes. Just as it does in the case of purely imagined "pure capitalism".