Yeah, there is no difficulty ramp up here. The only thing I could have thought of would be to spawn more fishing/hunting boats, but they're so easy to get rid of it didn't seem like it would work.
Well, yea, but say there's 3 fishing boats and no decoy sharks (or just one), you can't really stay still because hunger will kill you, but eating corpses makes you an easy target. Maybe balancing the two things would make it more challenging and perhaps more fun!
I decided pretty early on that when the fishing boats successfully kill a decoy shark, they ALL should assume they caught you and leave, just like what happened in the movie. So if all you have is one decoy shark, it doesn't matter if you have 2, 3, or 30 fishing boats after you. If I hadn't stuck to that decision, then what you're saying would have been a much more feasible option. I wish I hadn't done that. Your suggestion sounds like it would have improved the game a lot. Oh well, lesson learned.
It's okay, it was just my opinion, maybe others will have different opinions than mine, don't feel bad about it!
And yeah, game design documents are supposed to be always changing, maintaining the original idea is fine but mechanics should be changed if you notice they aren't working as intented or aren't as fun as you thought (imo)! :)