You can sort of imagine it as another generic "web component" framework, but blessed into full, native browser support, and getting some features/technologies that required browsers to implement new features to support them.
I'm curious about this. A graph of say, what percentage of the top 500 websites can't run without JS, over time, would be interesting.
My assumption is that it's a non-trivial percentage, and growing.
Does your site work without JS? If not, what justifies the more complex development and the degraded user experience for you and your audience?
The biggest issue I've seen with virtual-DOM sites is SEO. Google's crawler started using JS a number of years ago, but there are others that don't.
For sites that just convey text, they should work without JS. This would mostly solve accessibility (a11y) issues if sites looked to use simplicity wherever possible instead of adding complexity to solve issues caused by complexity.
On the other hand, these frameworks can be completely unnecessary overhead for one or two person teams. I'm currently using Preact for personal work, just because of how simple and lightweight it is.
To the author's point on site's that work in older browsers or without JS, unless you're building a product for government use or for an org that has really archaic IT requirements, the chances of needing polyfills that aren't provided by default in the framework are really low. The chances of someone needing your site to work without JS is also really low, especially if you're providing any sort of complex functionality.