I don't usually see MikeD because I have him ignored in my settings for trollish behavior, but this is an interesting point.
I did extensive lab work on EVSE designs with built-in AFCI and sag detection algorithms. My experiments showed that you would have an unacceptably high incidence of nuisance trips with various sag detection algorithms, and as Reg pointed out, AFCI alone is not adequate to stop most of these occurrences, though is does help occasionally.
For instance, the most common scenarios I've seen are simply high resistance at a single point, (or sometime multiple points) that over time, oxidizes and gets worse. Sometimes in only a few hours, and sometimes it may take weeks.
Then, once the resistance gets high enough, the thermal energy released is sufficient to cause ignition of many materials, thus triggering your catastrophic situation.
The "stab-lok" and other types of spring contact rear-wired outlets a particularly bad, as not only are the electrical connections relatively poor, but they also don't help conduct heat away as well from the (usually) worn contacts inside the outlet, thus exacerbating the thermal run-away.
If we only have a bad connection adding 100 milliohms, which I found was easily possible, that's over 14 watts of dissipation on a typical level 1 charging session! Depending on the composition of the materials near the fault, that can easily become a fire, and that's only adding a little over 1 volt of sag, so it's very difficult for a software-based sag detection algorithm to spot this and stop the charge session.
With the advent of cheaper pyroelectric devices and possibly even methods of performing active TDR pulse analysis, we may be able to catch some issues, but definitely not all.
The best thing you can do right now to ensure safety is to have a high-quality twist-lock outlet installed on a dedicated circuit and use that for your daily charging.
It's absolutely dangerous to use an existing, usually poor quality and/or worn 120v outlet for daily continuous loads, and it's more inefficient to boot.
For usually less than $600 total cost you can get a dedicated 240v outlet installed AND get your EVSE upgraded. We have a 100% perfect safety record with both generations of the Panasonic-made Nissan EVSE, and thousands of LEAF owners using them every day.
Don't be a cheapskate and put yourself at risk!
-Phil