There are a few practical issues with this massive stockpile idea. Firstly, this pandemic looks like it might go on for about a year rather than 90 days, which puts the stockpile size required to have one mask a day for everyone to more like 130 billion masks. With a five-year lifespan, that means producing about 2 billion a month, only a fifth the rate at which you'd have to produce them with no stockpile at all - except you have to do that all the time, whether there's a pandemic or not, and store, rotate, and dispose of all them. Secondly, you can't just dispose of the expiring masks by selling them to people who'd need them normally, because the pandemic-level demand is so much higher than normal demand that just refreshing the stockpile produces way more masks than we'd normally need. 3M's normal global production of these masks is apparently around 50 million a month, meaning that even your 90-day stockpile would involve continuously producing and disposing of about ten times the normal, non-pandemic demand.
Also, if countries bring their production home with some surge capacity, then this actually gives really great redundancy compared to the status quo because all the countries which do this end up with production capacity that can be used to sell supplies to other countries in the case of some regional crisis there. It's much better than centralizing it in the lowest-cost country as often happens now.
I figure that a 90-day (or so?) supply would buy us the time needed to build additional production facilities to bring us up to the necessary continuous supply.
Taiwan had a huge stockpile, and that let them increase their production from 100,000 N95 masks per day to 20 million per day while drawing down the stockpile. Asia has had success building these factories in ~14 days from pouring cement to startup, USA might need a bit more time of course.
Also, if countries bring their production home with some surge capacity, then this actually gives really great redundancy compared to the status quo because all the countries which do this end up with production capacity that can be used to sell supplies to other countries in the case of some regional crisis there. It's much better than centralizing it in the lowest-cost country as often happens now.