People throughout history have used things as reminders/mental crutches, whether it was nails set into candles to act as timers and alarms, abacuses to help with math, and so on. Also most people in the distant past didn't have to do as much reading, complex math, or mentally challenging tasks as people in the modern era often do. And people today receive better education than ever in history, and our descendants will get even better education yet. Maybe people will get slower at doing quick math in their heads, but there will be other things to focus on and learn. Intelligence isn't just about doing quick calculations in your head, so if technology can do all the tedious bits of work, it will give humans a better chance to focus on other things.