> most current browsers support very nice JavaScript features anyway
Maybe that's true for users in the western world. However, if you are in the asian market, there are popular browsers you've nerver heard of (some localized fork of chromium that's way behind). That is not to mention the poorer populace of the world still uses whatever that runs on WinXP or something, if they have Internet at all.
As software engineers it's not our job to use technology to dictate (control) how people access information, telling them to use newer browsers and newer hardware and what not, our job is to support as many people out there as we can.