Your decision on whether to comprehensively insure your old car will generally depend on:
its market value; and
your ability to cover the financial loss on your own if your car was, say, written off in an accident.
If your car has a low market value (e.g. $2,500), you may decide third party car insurance is all you need.
In other words, you may decide it’s not worth paying more for comprehensive car insurance as, over time, it could outweigh the cost of replacing your cheap car.
It’s up to you to do the maths: Figure out whether the lower protection and cheaper premium are a reasonable trade-off in your circumstances.