In reply to John Teague.

If there’s enough demand, plugins will appear to cover the non-backwards compatible changes

There is nothing easier in life than making blanket statements. No, it will not be possible which is why the move is done in the first place (some features can not be done with older browsers)

In the end wordpress.com is more likely to suffer from such a change, as people running a wordpress.org site will just not upgrade, but the wordpress.com team might get much more support requests.