You only have to subscribe to Unity for as long as you are developing with it and have made over 100k$ in the past year. You could in theory. Develop your game in Unity Personal, have a smash hit and make $1,000,000 and never pay Unity a dime if you quit development after releasing that first game. But in all likelyhood, you would subscribe to Unity Pro for 1 year, so you could at least patch that hit game. Now lets say it was a big, but short lived hit. You make a million in your first 3 months and then sales die off. After a year, you can cancel your unity subscription and continue to develop using unity personal because you didn't make over 100k in the last year even though your pro made game is still on the market.