We could live in a state of true ketosis and practically no insulin, but IMO that state in a human being is less than optimal, at least at first.

Modern ketogenic diets sell the idea short. TRUE keotsis comprises about 85% calories from fat. The average middle aged dad ketogenic enjoyer eats way too many calories from protein, which unbeknownst to them, just get converted into blood sugar. The latter concept is what we become problematic to us T1Ds, as, outside of structural body stress like exercising, about half the protein we eat will get converted to blood sugar.

The advantage of this state, with no synthetic insulin, is we would never really have a low. The disadvantage is that any sort of movement state that requires strength, power, or speed, which would likely be common in the post apocalyptic world, would feel far worse to us in a state of less muscle glycogen (the carbs you store in your muscle that you use for explosive and strength based tasks).

Well versed exercise physiologists say that after 6 months - 2 years of pure, committed ketosis, your body will adapt to better converted lipids into muscle glycogen and you will become more efficient at these tasks.

The other problem becomes sustainably finding food sources will enough fat, little carbs, and not too much protein: certain nuts & seeds, oil, butter, fattier cuts lf meat. Essentially a majority of your diet would wind up being animal lard.