If it's just the end-stop, you can by a replacement metal "zipper bottom stop", that you just crimp on with pliers. Cut the old plastic off, position the new stop and just squeeze the pliers hard. I'm sure there are youtube videos on the subject.
Important thing is size. There is sometimes a number on the under-side of the zipper pull, No. 3 or 5 usually. You can buy the stops on eBay.
Alternatively, you can as you say, use a heavy-weight thread, just wind around a few times.
Another method is to melt the plastic using a soldering iron, maybe joining two teeth, shaping it such that the zipper won't come off.
Bit of a shame to ditch a garment because of something so small. Also bit of a shame to replace the whole zip when it can be repaired so easily.