I think the game would be more fun if each swipe had a positive effect after the tower goes unstable. Because otherwise it just feels like a tedious task. For example, if the tower went unstable after a # of kills and the swipe made it so the next instability would happen at #+1 kills, that would be a great upgrade, that would make swiping unstable towers feel more rewarding.
It would also be nice to be able to send multiple waves at once, or spend currency on something more expensive. I didn't leak until wave 60 with 30000 currency and decided to stop because 40 more waves was more grind than I wanted to put in.