Once you hit a certain level of planes, they reduce the amount of money you make per second. They don't go any faster than the previous model, and they don't increase the amount per second. So by condensing your planes, you're not actually getting MORE per second, you end up getting less. And that defeats the purpose as the point is to condense to get the most money by getting higher planes. Sure, the cost more, like gems or coins every time you use them, but they don't give you anything in return, but rather take away. And why you're not actively playing, you get even LESS money than you would if you had it running actively.
I've read previous comments that say the money falls into the negative. The creators apparently fixed that issue, but when you return from the game, even after overnight, you've only collected what you would have gotten for about ten seconds of live-play, which is not the point. After a while, it's pointless to buy gems because upgrading the planes do nothing for you. The game is less fun than it was the first few days as a result. I'll probably use up the gems I already have and delete the game as a result.