I’m sorry but what? This is terrible advice. I have never understood the desire to keep the web “inclusive”. Is the web not already inclusive? Can someone not already build a website with simple html/css? The web has evolved and taken the place of desktop apps by and large. That is what SaaS has done. These websites are built to be fully functioning pieces of complex software. Using some tiny tools to markup html is…
"fully functioning pieces of complex software" - in my experience a majority of software is simply not that complex. And the few places where it is are not on the view layer. To make sure we're not talking past each other, can you give examples of the kinds of software you're talking about? We do quite a lot of react work too, but it's ~20% of the projects we work on, when advanced frontend interactivity is needed.
Even if you have a fully static website you generally have a navigation bar and footer that needs to be on multiple pages.
Things don't need to get very complex to benefit from frameworks and tooling.