They didn't retroactively changed the TOS of their engine so that a game you made years ago means you now potentially receive monthly invoices from Unity (and what about ridiculous new fees they will think in the coming years).

If Unity would've said that now any games built using whatever the most recent version of their engine is subject to a new TOS that means a fees per installs it wouldn't have nearly generated the amount of backlash it has received (no one is going to use Unity for a future project regardless of the fact that the fees per install is for the most majority a much better deal than 5% revenue).