My own memory and current experience on this point is that it used to be far more common than it is today.

I remembered there was a South Park episode where Cartman was being a stereotypical self-absorbed person walking around with their phone on speaker. I looked it up, and that episode came out in 2013. At the time, most phones on the market had a 3.5mm jack. Yet people not using headphones/headsets was an experience common enough to be turned into a joke in the show.

I don't think there's much correlation between 3.5mm jack availability and using a phone's speaker output in public.

"Simple" as you've used it is open to interpretation. I personally held on to wired headsets longer than most of my friends and family. You know what I don't miss, now that I've preferred wireless for a few years? Untangling the cable. Accidentally catching the cable on something and having an earbud ripped out. Picking lint out of the jack. Staying conscious of the length and positioning of the cable in the context of my own movements.

Other than the BT connection process, which is only complicated if you're fortunate enough to own multiple devices and headphones/sets to connect to them, wireless can be a lot "simpler" in actual usage.

I appreciate the counterpoint. The Cartman example is a good one. Also it's probably difficult to factor out the seemingly broken post-Covid social norms

One point I'll make is simplicity comes in many forms. Wired headphones can be dirt-cheap, they don't run out of battery, and I don't think they're as prone to getting lost