[chimeracoder]

That would just cause those users to switch to Firefox/IE/etc., which is not what Google wants.

If they're still using unsupported platforms, there's a good chance it's because upgrading is not a feasible option (at least not now/not easily).

[ocdtrekkie]

And people being forced to go to browsers that may have stopped getting security fixes even earlier is worse. It's not like you can simply ban an OS version from the Internet.