The truth about Inner Rim Width

Visualize the extremes, i.e. a rim that was "obviously" too narrow for a 2.1". Say 10mm internal. As you get narrower, the handling gets floppier, particularly in aggressive riding. The lie of the contemporary charts is they make this effect look binary where in fact it is progressive.

Mountain bikes started out with fairly rational rim to tire width pairings. In the 90s it got weird and one could in fact assemble a terrible handling rim and tire combination from mainstream product entries that were all nominally compatible and made for the same applications. For all their oversimplifying, the charts helped pull us back. Cyclists need simple.

There is probably a math-driven answer that deals with pressure, casing construction, rider load, and the forces imparted by different styles, situations, and terrain types. It would probably still be a gradient more than a binary, but the person who could really answer that might be able to find some areas where it gets closer to being a binary, i.e. if there are certain proportions in some situations where if moved beyond things get rapidly worse. It's probably one of those seemingly mundane topics in cycling that would actually take something like a graduate level physics class to crack into.