Too many firearms chambered in .40 S&W out there for ammo to go away in a lifetime.
Even .357sig is still out there and available for reasonable prices.
My rule of thumb is if there's at least 3 different major companies stamping it out, you're fine.
And I see ,40s&w on the shelves of every big box/sporting goods store I go to.
To me, what's getting harder to find is .38spl, .30-30 and .32acp. Not impossible, but pickings are slim and prices are reflecting that. I've got my preferences for brands for all these calibers and used to be able to wait a short while til I saw a good deal. Now it's either buy whatever is priced right or pay whatever they're asking for what I want.