Remember that you now have a situation where the vast majority of younger teachers do not know how to do mental arithmetic. As such, its use is likely to be discouraged, nevermind just not taught.
Students of mine regard my mental math skills as nothing short of magic, which is concerning.
Without mental math skills for rough estimates, they are frequently unable to spot when a 'switch-pigs' with the button-pressing has given a ludicrous answer.
Furthermore, the vast majority do not even know how to use many of their calculator's helpful functions (such as ENG or SCI display). They aren't taught it, and no one reads a manual these days.
That said, it is possible to train them if you can build their skills gradually on topics which are of daily relevance to them (e.g. how not to be late).