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.