there's a common joke among mathematicians, that as your mathematical mind develops, so does your arithmetic computer atrophy.

I think it's quite understandable; as a mind becomes aware of more useful kinds of numbers^[1], and more kinds of patterns^[2] that enable computational tricks, there is a temptation to explore new possibilities rather than charge forth along whichever computational path was learned at a younger age.

I honestly don't think that the capability of reckoning accurately and rapidly in decimal base is worth retaining at grade-school levels. Even if you're e.g. checking over a bill at a restaurant, the important tasks are probably remembering who ordered what and comparing the billed prices to the listed ones, rather than verifying that their point-of-sale performed arithmetic correctly.

1. most useful kinds are ideals or fields; the most familiar ideals are multiples of any prime, while e.g. "all ratios with a power of two in the denominator" is a field ↩

2. consider the trick of doubling and shifting the decimal point left, as a shortcut for multiplying dividing by five; it's only the first in an infinite series of similar tricks, for multiplying dividing by powers of five ↩