Regarding 'pulling the plug' at certain FPS threshold: interesting idea! That's probably how I would have solved this problem too. I'll keep this in mind for future projects :)
The decision to avoid hard lose states was definitely a right one, as trying to maintain balance between plants and animals in an unbalanced system would have made the experience much more frustrating.
Good luck to you with new projects! I'll be looking forward for you updating this idea, it is pretty neat.