Also, more speculatively I wonder if the food we eat is even worse than we already know it is and these studies are basically observing what happens when "poison ingestion" for lack of a better word is temporarily halted
In a mobile culture without access to lots of salt, there are few ways to preserve food. You eat what you find as you find it, as much as you can. Then you go hungry until you find the next big score. Hunters may go many days or weeks between large kills or a group reaches a more promising foraging ground, everyone scraping by on minimal nutrition like dried jerky and plants collected on the move. But doing that for a few weeks, or even months, is just fine, if everyone was well-fed at the start, anyway.
I know some people who have NEVER in their lives gone more than ~12 hours without a meal, and all of them are ~30 years old but with the standard array of contemporary american health problems more typical of people in their 40s. Our ancestors certainly entered ketosis at least occasionally. It seems like a drastic change to switch to never using that cellular process over a span of just 1 or 2 generations.