I don’t get it, either. Can’t developers just abide by their old contract and not update to newer versions of Unity?
Unity themselves have committed to that option (if you don’t like our new future TOS, keep the old version and don’t update) in writing (that was in their deleted github repo). So it seems extremely likely that they would lose in court.
The key words in the above are ‘in court’. If you’re an indie unhappy with an x*$.20 charge, chances are a lawsuit will not improve your day.
Not sure about the first part (you’d think so), but as for the second part, a lot of clients are set by default to auto-update. Don’t know about Unity, though.