> 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.