Very interesting, from a legal point of view
but if this is judged as illegal, all kinds of nasty effects will occur.
For exemple : what prevent oracle from changing the licence of Java and ask for royalties for all java based apps after a certain date or something ?
or microsoft from asking royalties for apps using DX ?
etc
i know they can't change an active licence, but they can change the licence starting a certain date, and applying it to all apps created after this date.
And even if they swear they won't, legally it wouldn't be mandatory to do so.