I'd wager it won't affect most small studios, since most games never get close to the prerequisite $200k revenue in a year. But the thought that if you finally do see success you have to start paying danegeld to Unity...?

Far worse is that Unity apparently has the ability and desire to do things like this. What's to stop them in the future from altering the deal further?

You already had to upgrade to Pro if your company (now it's per game which in theory might be a significant discount) had more than $200k in revenue. So yes, but you still had to earn over 1 million per game and the fees are not that bad with Pro (since you get 1 mill for free).

Unity will still likely be much cheaper than Unreal. Zero respect for customers and unpredictability (what they might pull off next?) would seem like much better incentives and cost for most.