So... even if it's English-only, I can't quickly refer to a price in £ or € or ¥? Or that it's 90°F out? I can't talk about my fiancée? I can't even use “proper quotes”?
Yikes.
These days, talking about the "benefits" of ASCII-only is like talking about the "benefits" of HTTP over HTTPS, or the benefits of dial-up as opposed to broadband.
No thanks. It's exclusionary and for zero good technical reason. For every reasonable programming language, there are functions to do everything in UTF-8 that you can do in ASCII, with about the same complexity.
In 2020, why are you going out of your way to prevent people from typing the honest-to-goodness useful characters they want to communicate with? In English?! Where exactly does UTF-8 not work, that this site does?