Full stack developer here who disagrees. Most web apps are not that complicated and if there are the term 'backend developer' will be replaced with something more specialised as well.
> the backend story for me is still weaker than using Python or elixir, etc simply serving an API
That is a very odd statement, Python has a lot of issues that NodeJS never had, in terms of developer experience I believe NodeJS is superior just based on the fact that I don't need to know what a virtual environment is and I don't need to choose between different incompatible package managers before I even write my first line of code.
> if you are building a company, 9 times out of 10 you are going to have to have an API anyways. I like to have the boundary of that API explicit.
If the only consumer of the API is your frontend, why would you want to have such a strong boundary?
> the only exception to the above is if you aren't building a company and instead working on a small personal or group project.
This statement can easily be hard countered by the rise of full stack frameworks like NextJS and the fact that the whole backend-frontend separation didn't always exist, it mainly happened because of SPAs.