First, a clarification: I understand the aerodynamic reasons that interstages are used, and I understand that the shockwaves that would be generated without an interstage would wreck the rocket. My question is if there are any non-aerodynamic reasons an interstage is required.
My contradictory thoughts:
The engine bell(s) already support the entire compressional stress generated by the weight of the stages above it when the engine is firing, and does so in a controlled manner--the gimbals don't fail to keep the firing stage pointing in the right direction.
However, the burnout Gs of the upper stage might be fewer than that of the lower stage, so the nozzle will not support as much weight as the interstage has to.
Also, the aero forces might cause the compressional stress upon the interstage to exceed that which the nozzle can support in low atmosphere.
However, max-Q is usually well before burnout, so total stress even with drag considered is likely still less than it'd be at max-Gs.
Replacing load-bearing interstages with much thinner ones that serve only as an aerodynamic surface would save a lot of mass.
Or even allowing the nozzle bell to take part of the load seems like it'd allow you to save a lot of mass.
But it clearly hasn't been done yet (I think, related question here), so there's some "nail-in-the-coffin" reason that interstages must be entirely load-bearing.
Finally to the question: Why aren't nozzles used to bear the stress of accelerating upper stages? Why are interstages used (required?)?
Am I misunderstanding the stresses involved? Do the stresses during engine burn differ substantially from those endured when being accelerated by the stages below? Or is it an orthogonal issue to the stresses of launch--maybe it's difficulty designing an effective nozzle which can be reliably decoupled from the stage below it?