Now suppose another wire were added from the battery negative terminal to the frame. Now if the negative battery cable connection were to fail, the return current path would be entirely through this added wire. If it is a much smaller gauge it may overheat/melt/catch fire/etc.
I certainly can't disagree with this as your logic is correct. If at all possible find another large braided cable and use that to connect to your frame or, in the very least, use a fuse in the wire from the battery to the frame. It may blow if your starter ground isn't up to the job but at least you'll know why.
Point is to maintain your electrical system as best you can. Even if not in doubt it's a good idea to clean everything and every connector then put some electrical silicone grease on everything. I have that job facing me when I get mine back on the road because, as mechanically adept as Old Okie is, he's not totally educated in the electrical part so I'll probably do it out there so he can see what's involved. Same thing goes with the painting which we'll do out there, kind of anxious to teach him how easy it is to do 2K Urethane given proper prep, mixing, dry air and the right weather. By the time we get the right weather around here most will have already plowed their fields thus the dust will be out of the air.