I wonder if this process would work if the water were replaced with milk. Milk would degrade at high temperature, but this (at least in bulk) is at room temperature.
Sometimes I wish HN had flags of where the person is living next to the username or something. This is obviously true for some/many places in the world, but also probably sacrilegious to even think out loud in others.
FWIW, once you poor milk into your espresso, it stops being an espresso, and depending on the coffee/milk ratio (and milk preparation), you know have a Cortado, Cappuccino, or a number of other versions.
So ultimately: no, no one drinks espresso with milk, it's not a thing, they start with an espresso, then turn it into a different espresso-based coffee drink :)
Really? The Last time I was in an American office, there didn't seem to be any milk around. They had an unrefrigerated bottle of "creamer" next to the coffee machine, but I was too scared to try it. I'm still not sure what it is, or how I'm supposed to use it.
Creamer is a liquid made with oil and sweetener that approximates the taste and mouthfeel of milk and sugar. Or really a flavored syrup, not sugar. They often have hazelnut or vanilla flavoring added. You would use it by pouring it in your coffee, at about a 1:1 ratio with how you would use milk.
I'm surprised you associated it with espresso. Creamer and drip coffee machines go together. Milk and espresso machines go together. I've seem drip coffee machines with real milk, but I've never seen an espresso machine with just creamer.
It tends to exist primarily in office environments, because it can stand not to be refrigerated and was the first lactose-free option. Therefore it was easy to buy one bottle and solve the problem of "stuff to add to coffee".
Office coffee is not what people drink for pleasure -- it's a caffeine delivery device. If you go to a coffeehouse in the US you'll find that people often order drinks called Lattes which is espresso and steamed milk. And the word is Italian like espresso itself, so I think this isn't some weird Americanism but pretty traditional.
Espresso is so named because you "express" the brew from the beans and you do that with water because water is pretty neutral in flavour, is not poisonous and has quite a few other properties that we have evolved to exploit or live with.
Milk is a weird liquid associated with mammals nursing infants. We humans have evolved to be somewhat lactose tolerant post infancy which is rare in animalia (1)
Given that we are using the Italian word - espresso - then let's use their definition. If you add milk then you have a latte or a cappuccino or an americano con latte, a flat white or whatever.
Real weirdos try to milk oats. I've tried but I can't find their teats.