I do not think we "lost" our ability. I think we changed our thinking.
I think the world has mostly accepted the "good enough" versus "perfect". As we all have heard, to become an expert, on average we need to pursue the subject for 10 thousand hours (? Malcolm Gladwell). But, we do not need to spend 20 years practicing. We can obtain "good enough" in a few weeks, or even few hours (obviously depending on the subject).
For example, to win the Grand Prix de la baguette de Tradition Française de la Ville de Paris (i.e., French Baguette Competition of Paris), many spend a lifetime perfecting their craft. I can teach you in a day how to make an edible baguette that the average consumer will enjoy.
I think our "attention span" has shifted to "good enough" in many instances. I do not think this destroyed our attention span capability, it just made it slightly different.
Final anecdote to "prove" my point we did not lose our attention. I have taken ADHD-diagnosed boys to camp and fish. Of the twelve (ages 12-16), only one could not sit patiently and watch the line and bobber for extended period. He became bored, and started whittling for the same amount of time. Once they returned to "civilization", they "became" ADHD again.
As someone already noted, in my experience humans cannot multitask. We can context switch, some very slowly, and some very fast. But, we do not multitask.