It's very common to log when a button is clicked. Which means we can already reconstruct a user's session because we know what coordinates buttons have. We might miss out on the random movements and such, but that might not be a big deal if we also log the timestamp when a button was clicked.

Again, if I designed my site to use a websocket instead of event handlers - the result is the same. Are we proposing we outlaw this? Or inversely, placing in law the only accepted method of website design? That seems foolhardy to me.

Your car collects huge amounts of data about your driving habits. A Tesla, for example, might auto-report that data, but your "dumb" car also provides that data to an ODBII reader. In fact, that was the premise for those Progressive (I think) Car Insurance things they wanted you to plug into your car to "prove" you were a safe driver. Any time your car is in for service... this data can be freely read. Your insurance company will also obtain this information after an accident in some cases as well.

We use mouse/key movements for a lot of things, not just reconstructing sessions. Google's newer "I'm not a Robot" captcha's just ask you to click a single button for example. A ton of anti-fraud/abuse systems do this as well, particularly for credit card checkout forms (anti-bot measures). The ramifications of a bad decision for this suit are huge.