The only reason people will continue using Unity is because they've already made )or are in the process of making) a game using it and switching to something else would waste massive amounts of time and effort. Unity is depending on this - this is basically them squeezing everything out of existing customers without regard for long term growth.

Remember, the whole idea here is that Unity is demanding payments for already existing games. They clearly don't care about whether people keep using Unity for new games in the future; the executives who made this decision will have cashed out and will be long gone by the time all the existing Unity games in the pipeline are done and things dry up.