It expires. I don't know at what point it becomes unhealthy/dangerous but infant formula is one of the few shelf-stable items that's illegal to sell past the use-by date. Many things are perfectly safe/stable past their 'sell by, best by, use by' dates (bottled water, most canned food) but I haven't read data on this specific item.

Who is we? You are free to stock your pantry as you desire.

What are we going to be short of in 5 years' time, that we should start stockpiling now?

Sure, it may not be possible to stockpile every niche product, but this is literally food, perhaps the most obvious thing (up there with water and medical supplies) for governments to stockpile.

It's food for a specific niche though. More broadly, the US government does have a strategic grain reserve, holding wheat, corn, sorghum, and rice. Or at least, there was one, until 2008 when food prices spiked and the grains were all sold off. The reserve, called the Bill Emerson Humanitarian Trust, continues to exist to this day, but only holds cash/securities and no longer holds grains/commodities.

You mean the niche of babies?

Pretty much everything?

Lean does nothing but put cash in the pockets of those at the top at the expense of everyone else when there is a hiccup in the supply chain.

No, it makes the stuff cheaper.

It's easy to see in hindsight that a certain item should have been stockpiled in anticipation of a shortage. What's hard is predicting what's going to be a shortage years into the future. Petroleum is an easy choice since energy is almost always in demand.

And then there's cases where there was good foresight in stockpiling certain essentials in some wiser countries, but then they go and decide to stop stockpiling. The craziness! ;) (see https://www.bbc.com/news/world-europe-50402048 https://www.economist.com/europe/2019/11/21/defending-switze...)