One other thing to consider is this. Usually the switches, fuses, etc. in a vehicle are all on the POSITIVE side of the device being powered by the vehicle electrical system, not the negative. If the dangers of direct connection to the negative battery terminal were at all that problematic, the fuses and switches would be on the negative side.
That's just silly. :-)
The problem has nothing to do with where fuses commonly are.
The problem is the negative buss in the radio comes out on every grounded pin or connection, including the case and antenna. This means if the negative post of the battery to vehicle chassis comes open, the vehicle current returns through the radio lead. Since the fuse is normally many times larger than the radio's internal ports might handle, or foil traces, and since a fuse takes time to open, a negative battery open can damage the radio....negative fuse or NOT.
The entire concept of connecting to the battery negative lead is **idiotic** from a design standpoint, unless the fuse can be faster and weaker than the path out ANY radio port and the radio manufacturer (or whoever tells us to connect to the negative post or lead) can assure everyone whatever connects to the radio on any radio port is significantly stronger than the fuse.
I can give an example of this problem. My IC751A has an open keyer paddle ground lead because, while the radio was in my truck, a negative radio fuse went open. The key was grounded through a random metal contact, and became the primary path for radio current. This opened the foil trace deep in the radio.
The same would have happened to a mic lead, if the mic were in ground contact, after that fuse opened.
A similar destructive failure can happen if the battery negative goes open, which can then apply positive potential to grounded jacks on the radio.
The entire concept of a negative fuse and a negative connection to the battery at the battery lead or post is just totally bizarre from an engineering standpoint. I think the idea probably came from people not understand the other end of the system, and then just repeating a bad idea.
Right from that we see that logic doesn't always hold true.
Your concept was wrong.
I do understand the problem about negative backfeeding in case of ground failure, but that is why installation instructions specify the negative lead from the rig be fused, negative rail isolated rig or not.
That silly fuse idea only works if EVERY port exiting the radio can handle more current than the fuse, and handle positive voltage. Anyone who thinks a 20-25 amp auto glass fuse (that typically takes 30 seconds or longer at 2X current to blow) will pop before a foil trace or small component is damaged, or before any wire or lead might overheat, is just not thinking clearly.
Now if the case had a solid hard ground it would save the radio and wiring, but then the lead to the battery and the fuse path would be non-functional because the resistance of that path would be significantly higher than the chassis path resistance to the battery.
The whole "lead to the battery negative system" is an example of pure unvarnished ignorance in electrical planning.
One last thing, let's remember the actual power flow is from negative to positive, not the reverse.
What does that mean?
First, it is electrons that move from more negative to less negative (more positive) potential. But that has absolutely nothing to do with "power flow", and has nothing to do with fusing.
A ***proper*** fuse application would prevent fire or major damage by having the fuse protect the path properly, regardless of polarity. Ground leads are should NEVER be fused, unless the thing being fused is totally isolated from external paths or contact. This is especially true if the fuse could easily be bypassed by external connections.
This negative fuse thing is like a bad riddle. If we make the system safe by solidly grounding the radio case (and negative buss) to the vehicle chassis, then we almost certainly have much less resistance in the chassis to battery path then in the radio wire to the battery. If that is the case, and it almost certainly would be unless the battery to chassis had a high resistance defect, then we ran a useless wire to the battery.
On the other hand, if we 100% guarantee to FLOAT the radio and every radio connection from vehicle chassis, then the negative fuse system is safe and we can use it. So we go though extraordinary effort just to make a higher resistance path to the battery than a chassis path would typically have.
Now there can be exceptions to this, but the exceptions are rare compared to the much more common system where the chassis is a better more reliable path.
We don't see the vehicle manufacturer running his stereo system, lights, horn, ECM modules, or anything else back to battery negative and fusing the negative. Now we might get into systems that do that with hybrid vehicles, but standard vehicles are not built that way.