I don't understand at all why this would be unethical. Maybe I'm working for a company making a product I have expertise for and I'm passionate about, but they're making decisions that I feel make it worse for users unnecessarily. I leave and start my own competing product to do it my own way that I consider to be the better way. This isn't unethical. This is the nature of all competition everywhere going back as far as anybody can remember. I've stolen nothing at all, so I'm not sure on what basis it would be unethical.

And if it's a topic or industry I have expertise and knowledge of, why would I throw that all away just because I worked for a company where I used that expertise and knowledge? That's basically implying that we're all "ethically" subject to non-competes for the rest of our lives after working for a company making a particular kind of product.

That's clearly and absolutely "inelegant", to use your phrasing.