MAD ammeter bypass question

I basically agree. But will say that the main purpose of the fusible links is to deal with the battery dead shorting. The battery will flow well over 40 amps when dead shorted. Heck, people can and have arc welded using a battery!
The ignition run wire (J2) from the key switch is usually 16 gage. People should not be expecting fuselike protection in that circuit. If it shorts, then engine stops running and the alternator rotor stops getting fed so either way its going to shut down. I suspect that's why the manufacturers focused on the battery dead short as the main thing to be concerned about with respect to the main feeds.