I think you are being too pessimistic about the speed of mass adoption. On the desktop: Firefox, Chrome and Edge auto-update. On mobile: Android 4.4+ (over 80

I think you are being too pessimistic about the speed of mass adoption. On the desktop: Firefox, Chrome and Edge auto-update. On mobile: Android 4.4+ (over 80% and rising fast) auto-updates its webview component. iOS receives yearly updates that are applied by a very large majority - though Apple's browser development does seem to have stagnated a bit. So it seems to me that non-updating browsers are going extinct, and that we won't be dealing with another IE6 anytime soon. We're living in the future! :-)