They definitely could have added a variety of methods to indicate that a tool would soon break. Visual queues (think chipped axe from previous titles), audio queues, User Interface Indicators though this one wouldn't really 'fit' in well.
First one I mentioned definitely could have been done:
- Axe could have chips in the axe head.
- Net could start to get holes in the netting.
- Shovel could have a chipped shovel head, or the shaft could be cracking.
- Fishing rod could have cracking in the rod.
- Watering can could rust slightly.
- Slingshot could start to crack as well.
But the reason why they likely did this was because of how many different tools there are, and variations to these tools there are due to customization.