If you get a million dollars in revenue as a solo dev, it's pretty much just winning the lottery. You've already spent all the working hours to make the game, so it's pretty much all profit at that point. What costs do you even have?
It's a free choice to use the engine, you can use another engine or make your own if 5% is too much for you.
You typically only win the lottery once as well... It's 1 mil at the release date of the game and the following maybe 3-6 months. Game sales usually happen at the release date and then there's a pretty much insignificant "long tail" of sales.
Anyway, while 1 million isn't really enough to retire instantly, for sure it lets you retire 10-20 years earlier if you're just a little bit smart with money.
It's a free choice to use the engine, you can use another engine or make your own if 5% is too much for you.