You only have to handle the 20% of users as a special case if you have gone and made some technology choices that excludes them. There is nothing inherent about the internet or web that says that needs to be the case.

A rephrasing might ask why we make technology choices that exclude 20% users?

If we're not able to produce a single page web app that reaches 100% of users, that's cool... just make a many page web site without lots of JavaScript that does.