Measuring a fuse for open is easy-ish. Replacing it in the DC-DC or Charger isn't.
I'm not sure what you're asking me. I'd said that fuses don't "go bad" or "get weak", two phrases that I've heard a lot over the years, by people who don't understand how fuses work and seem to be genuinely confused by electrical devices in general. You're asking:
What other reason do you see for such fuses blowing, if not exceeding their maximum allowable I2T value?
It seems we agree: fuses open because a current was exceeded for a long enough time interval; that's the purpose of fuses.
I wrote about fuses not "going bad" because I write that every year or so reflexively, whenever the subject of fuses "going bad" for no apparent reason comes up. Absent physical damage, fuses blow due to a circuit overload, not "it was old" or "magic is afoot in the world" or whatever superstitious reason a person might ascribe.
In the old days, fuses blew a lot more frequently, and equipment was designed with that in mind, so fuses could be changed easily; they were "user serviceable parts" in the old parlance. Nowadays, it seems fuses are buried many man-hours into equipment, and what purpose they thus serve is no longer apparent. To protect the wiring? If the fuse is not serviceable, that is (I guess) the fuses' only purpose now.