> would you swap a competent employee for an incompetent one, paying the same amount?

But this is the glass-half-empty boss. The glass-half-full boss is happy to have a competent employee who can be called upon when needed for important front-end stuff and is picking up backend work at a faster rate than the intern the team would have otherwise, because they're already an experienced engineer (with knowledge of the front-end no less!) and there is skill beyond just writing code that transfers between domains.

And that's to say nothing of swapping a competent employee who's already onboarded for an empty seat and a talent search, when the employee quits to expand their skills elsewhere.

They’re not incompetent. They have domain knowledge in both the general industry of the company and the fine details of the company itself. They’ve probably been nearby the backend developers while they were talking through issues and even offered insight in the lunch area about specific algorithmic problems some backend developers were having.

Languages tend to be super easy.

Frameworks are a little bit of work.

Domain knowledge is hard.

If that competent employee wants to switch career tracks and you won't facilitate that, they'll just go get hired somewhere else that will.