Let's say masks provide 100% protection to both the wearer and others (perhaps non-masked individuals) in close proximity. This is perhaps the worst scenario.
The immune system requires a consistent influx of both microbes and viruses to remain trained.
Halting the exposure to microbes and viruses is extremely harmful in the long run and leaves you exposed to enormous risk in the future.
Without training, you will become susceptible to even weaker microbes that your immune system would have normally handled with ease.
This is no different than laying in bed for a prolonged period of time. Walking up stairs, an activity that normally would not be a challenge, would all of a sudden become difficult.
Your immune system would slowly degrade similarly not able to complete previous tasks.
This is particularly harmful to children who's immune systems do not have the training of adults and would leave them more vulnerable. Children have the most life to lose so it harms them the most.
There is no end game with 100% protection. Everyone would have to continue wearing masks indefinitely as immune systems would continue to weaken and respiratory viruses are unable to be eradicated.
This scenario also makes it more risky for the susceptible for longer periods of time since the healthy do not gain immunity. And unless people wear masks at home (the most likely place of transmission) there will be a breach and the susceptible will get sick.
This scenario increases fear and anxiety. If you knew something offered 100% protection, any encounters with someone without a mask would trigger the fear response within you.
The fear and anxiety would only increase as your immune system weakened.
This is a viscous cycle of more and more extreme measures until everyone just becomes bubble boy.
So the scenario with 100% protection leaves you fragile physically to microbes/viruses and fragile mentally to being around others without a mask.
All this said, I don't think masks provide any protection. Microbes/viruses have evolved together with animals on earth for hundreds of millions of years, competing and strategizing with one another for survival.
All mammals use breathing, spitting, coughing, and sneezing to expel dust and germs from their respiratory tracts. All of these mechanisms are impaired with a mask. So, if harmful bacteria or viruses do enter your mouth, it becomes much more difficult to expel them.
Masks are actually good at collecting bacteria as they are 100x larger than viruses. Humans never evolved to keep a warm moist environment of bacteria trapped in front of their face for an extended period of time, breathing it in deeply breath after breath
Normal breathing, coughing, etc... expels this bacteria away from you.
Masks are competing with hundreds of millions of years of evolution, preventing you from completing the most simple task we have to expel germs.
Masks appeal to the brain, because like duh obviously, they put a barrier between you and the germ. But, there are many counterintuitive results in science.
Clogged arteries are filled with cholesterol, yet eating cholesterol does not clog your arteries.
Overeating junk food, seed oils and refined carbohydrates along with elevated stress leads to metabolic disease and overall body dysfunction, which leads to poor arterial health. Cholesterol is there to repair the diseased arteries and is found at the scene of the crime.
No different than seeing police at the scene of a crime and accusing them.
Mother's breast milk is filled with cholesterol and your body will produce the required amounts with what it does not get from diet.
Another counterintuitive result - the war on drugs - how do we stop people from abusing drugs? Put them in jail!
No. This exacerbated the problem. Removing all penalties for users and providing them resources to overcome drug addiction is what works.
Just like the wars on cholesterol and drugs have failed and been exposed as extremely harmful to society, so too will masks.
There are many unintended consequences to mask usage.
