Still, a seasoned C developer of 10 years will not be as good as someone with 10 years in front end development experience. And vice-versa. That's the OP's poin

Still, a seasoned C developer of 10 years will not be as good as someone with 10 years in front end development experience. And vice-versa. That's the OP's point. You can substitute any technologies there and this will be true, all else being equal. It's not just about learning another language but also learning tools, libraries, systems, etc. and having the experience in a specific domain. What the OP describes is how just about every single company works. Is it ridiculous? Partly yes, partly not, but ridiculing it won't make it go away. The tech can be picked up, though even that takes time. There's no substitute for experience.