Game is fun until it gets to the saws in level 14. From then on your income decreases by around 80%, so you can't get more upgrades and the game actively punishes you for playing, with the only thing you can do being staying in the corner while whatever auto-circles you ended up buying farm a tiny little bit for you until the boss shows up.

If you didn't buy any auto-stuff you're completely out of luck, and even if you did buy some, the game is still slowing down by a lot.