Thanks for awesome response Griddolini.
+1 to updating mouse mechanics.
You should probably require crops to progress in the game/survive, in some fashion or another since they aren't useful now.
I think this would be a fantastic change. And the explanation you gave was great. Also, if we did a time-based constraint on the game with rounds, it might make the player feel like they're actually progressing. The only "progression" we have is score and slowly increasing amount of aliens.