I wonder if these advocates actually have children or have interacted with children.

I have a 15yo son, and I can confidently state that he quickly forgets skills he has show proficiency in when they aren't practiced regularly. Yes, there may not be much need to make students take their long division skills to the max with massive numbers, but there definitely is value in having the method firmly entrenched for the times when they are guaranteed not to have a calculator (or phone) handy. And mental arithmetic should be a lot faster than grasping for a calculator (and definitely faster than grasping for a phone, finding a calc app, launching it). It personally makes me a little frustrated and sad when people can't do simple arithmetic - and I have to end up waiting for them to find a calculator to verify the simple sum I've just done in my head, eg when receiving change from a vendor.

We don't all have to be adding machines, but it would be really nice if the average person could do basic arithmetic without having a panic attack because they're nowhere near a phone.