I actually thought about this the other day when a bartender was trying to make change for me. They got confused when I gave them $60 for a $39 tab so I'd have cash for tip, and I was explaining "my bill was a bit under $40 and I gave you $60, so you should be giving me a bit over $20 back". And it occurred to me that it's the sort of estimation you learn when you don't have a calculator on you at all times.

I never had to work with a slide rule, but I learned from engineers who did. But I didn't have a calculator on me at all times, so I still had to learn estimation from people who were masters at it even though I myself didn't need to be. But I don't have the same level of mastery of the skill as those who taught me because I don't need it as much. The generation who learn it from me won't need it at all. Yet even though they don't need it, something is lost by them not learning it, the same as something was lost by my generation learning it but not mastering it.

I'll guess we'll find out if this all matters if planes start falling out of the sky more often then it seems they ought, because no one looked at a result from the computer and thought "hey that doesn't feel right".