Earlier quoted context omitted.
The 500 people browsing internet without JavaScript enabled by default would have to enable it. Difficult times.
I'll avoid getting into the whole discussion about people on slow connections, accessibility, etc., as I get the impression you really don't care about them, and just point out the obvious fact that everyone has JavaScript disabled whilst your JavaScript is downloading.
Slow connections are not a problem, because React is ~40KB and Preact is ~3KB. If downloading 3KB is too much for your network, it's not the fault of React, JavaScript or any web developer that you're gonna miss out. Also, server-rendered HTML would NOT be smaller than 3kB - in fact it would require MUCH more bandwidth since server-rendered HTML can't be cached as easily, so every page will re-transfer the exact same HTML for shared things like navigation bars, footers, etc.!
Accessibility isn't any issue either, this is a ridiculous argument to even try to make, accessibility tools use HTML and are fully aware of the existence of JavaScript - React doesn't magically bypass HTML to display something on the page.
> everyone has JavaScript disabled whilst your JavaScript is downloading
Everyone has HTML disabled whilst your HTML is downloading.