> Consider the nuclear reaction metaphor. It's clearly not memoryless. Eventually you'll run out of fissile material.
I think no real process is memoryless: time passes/machines degrade/human behaviors evolve. It is always an approximation that is found/assumed to hold within the modeled timeframe.
Sure but it's still the case that if you treat the reaction as memoryless, your model's predictions will be wildly wrong. It's not a good model in this case.
It seems that most approaches treat neutron diffusion as memoryless (or at least stochastic):
https://en.wikipedia.org/wiki/Neutron_transport