But over time, people will become more susceptible to it due to weaker immune systems.
Yeah, not how this works.
Infection rate is the same, weak immune system or not.
(Oh, and while we are at it, weak immune system is nothing thats a universall thing someone can just have, or that someone gets, if they dont go outside often anymore. (There are allergies, there is aids (/auto immune illnesseses), and there are people producing less antibodies, for this virus, but not for another, and we dont know why.))
Lets say your point were correct, and staying home, made people sick with different illnesseses, thereby making them at least more susceptible to dying (infection rate still would be roughly the same). That percentage - would still be so low - that people carrying the virus around, sometimes with no, or only mild symptoms is the bigger issue here, IF your medical system is on brink of being overtaxed, which in the US it is.
So the more people carry it, the more people get sick, the more the health system is taxed, the more people die. 'Weak immune system from people getting not enough sun' doesnt compare risk profile wise.
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Staying home works as many people are getting over the virus at home, or in hospital - but at the same time, stopping the exponential infection rate. While people are on curfew. And lowering the overall number, quite radically, as you prevent further spread, while people get over it.
If you then open up stores again, even if the infection rate becomes exponential again, but from a far lower case number (people got over the infection in quarantine), even exponential growth is less worrying - for a time. If numbers become big again, same procedure, or hopefully some sort of treatment becomes available.
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Now here is where you are right. In certain businesses (gastronomy) losses now are losses that dont get 'reconsumed' after a curfew, jobs lost now, doesnt mean, that they will come back when curfews are lifted, and unemployment rises throughout the US.
So ultimately curfews will have to be lifted as a result of that. That doesnt mean, that peoples consumption behavior will be back to normal right out of the gate.
And at the same time, if you press on doing it faster, infection rate in the population is higher, and you've gained not as much as if you could have, if you kept curfews longer.
The next time your hospitals become overfilled people will be less likely to comply to curfews to 'fix that' anyhow. So why not give it at least one good try?
Its not like someone could overlook, that the economy doesnt like curfews. And its not like someone wouldnt have thought of that unforseeable effect, of people getting less well the longer they are isolated.
Its just mainly, that all those fruit juices, and yoga lessons, and fitness centers really have no effect on your immune system at all (medium term (as in months not outside), and its absolutely the right thing to ignore your theory of 'but more people will die from not jogging enough'.