Just yesterday we were talking about Discord (on Discord) @ job. I said something like "discord might be convenient, but searching for things is difficult in comparison to IRC". Then people asked me "what is IRC". Hit like a ton of bricks.
I recall reading an article or something about accessibility a while back. The author had commented that they saw someone coming out government forms on a Nintendo DS (or some such) and that person had commented it was the only thing they had.
I think we developers grossly underestimate how people access technology and what our minimum targets should be.
For something like a government website, I agree. Access for everyone (except maybe a really unreasonable case) should be a priority. For the average web app, I think it would hold the developers and the users back. If half a percent of the web can't use your SAAS, that's fine. If you're developing a product initially - go for the masses.