There, I just told you to upgrade without sniffing UA.
It is ridiculous for every website to be babysitting their users software, when we already have operating systems that keep software up to date, package managers that keep software up to date, and software that keeps itself up to date.
If I need to make a really important website that can't lose users like that, I just need to make damn sure it works with any relevant browser without any UA sniffing hacks that will bite back down the line.
If I'm making some bleeding edge crap that really needs a bleeding edge browser, I'm sure I can find a place for that sentence where most users will find it, and if I lose a few users, it probably isn't that big of a deal. Chances are I'd lose more users anyway due to many not wanting or being able to update for any number of reasons.