Yes, this would be true if the rarity of someone who could write front-end and back-end code in proportion to the number of software developers was as rare as t

Yes, this would be true if the rarity of someone who could write front-end and back-end code in proportion to the number of software developers was as rare as the number of people who could jump 7 feet is to the number of people who can jump. It's more akin to extending the three-point line between college and the NBA. While extending, you say "but nobody can shoot from that far away" while NBA hopefuls are draining shots just out of your view. Sure, the line is too far out for college, but there are _plenty_ of professionals who can shoot from that far out who are both in and out of the NBA. Sure, nobody at a charity obstacle course will be able to perform at the same level as a professional. Good thing we're talking about professionals software developers when we talk about full-stack developers and we're not talking about professional nurses or amateur painters. It's rare within the general population, but it's not rare within the world of software development.