To me, it actually seems absurd to claim that people can only be great in one thing. The real problem is in thinking that "designer" and "developer" are things

To me, it actually seems absurd to claim that people can only be great in one thing. The real problem is in thinking that "designer" and "developer" are things. They're not. They're a massive collection of skills, techniques, practices, etc. Two groupings of which we've drawn arbitrary circles around and applied the labels "designer" and "developer" too. Those labels aren't real - we made them up. We can draw different circles around different groups of skills. If we want to.