They were right, though. For one, you still need to now what kind of equation to put into the calculator. If you don't know the concepts, what are you going to calculate. Real life situations don't actually spell out neat textbook questions.
And just take a look throughout the day how many quick maths you do subconsciously. Just things like cooking or grocery shopping have tons of moments where you just do the calculation in your head. And you can do that because you've learned how and when to do that.
It's not just a matter of "What if it's inaccessible", but also "Do you want to be dependent on your phone even more". Are just going to walk around with your phone on voice mode narrating your life into your LLM to let it tell you what to do?