I don't think it works like that, from my recollection of the uni courses I did 20 years ago.

Even a small advantage like 1% will quickly propagate in a population, because it's about advantage over 1,000s of generations.

That this disease defence CAN be turned on, means some people would have at some point had a genetic mutation to turn it on.

As the GP pointed out, therefore it must be a net negative from an evolutionary stand point.

I also suspect it would be calorific consumption, as someone else said, so it might be ok.

However, there are plausible other explanations. For example there are medical conditions that result from a too aggressive immune system and it could instead be reducing the chance of that occuring.

I would say you are both right in that if you have two competing variables (on-time for the defence vs calorie consumption), when the main causes of death before procreating were infectious disease and malnutrition before modern times, I would expect some equilibrium to be reached and we have not had that much time to evolve since caloric scarcity in the western world was a solved problem for large swaths of the population.

If in the future we could trade a few hundred extra calories per day for a great immune system (without auto-immune side effects) we would have found a nice cheat code!

Thinking about your point- I bet we do not know if some people have it on or not. It feels like something that would have to be specifically investigated.