I always figured ditching the 3.5mm port on the phones was about IPxx ratings and water resistance.
If they were touting something about audio quality I missed it. But since I can't remember the last time I used a wired headphone connection on laptop or phone, it's no surprise that I paid it no attention. *shrug*
Still doesn't change that Apple's pretty clearly maintained the client/host paradigm I mentioned and putting Lighting into a mac would go against that.
Perhaps. Given how many still today complain about interfacing their legacy cables and devices to the MBP/MBA USBC ports, switching from USB Type A ports on computers to Lightning back in 2012 would've been sheer folly.
Looking back, it seemed Lightning was aimed at avoiding the "sidedness" inconvenience of micro-B and mini-B connectors. As you remember, the Type A connector was/is largely a host-side connector, with the Type B connector (in different flavors) used on the client-device end.
It was not about IP rating as other phones with the same rating had 3.5mm jacks at the time. Audio quality is definitely (potentially) better as it is a digital output on Lightning vs 3.5mm. The D2A in the iPhone isn't that good because it has to be so small. You can find plenty of USB D2A used by audiophiles, but realistically it wouldn't take much to be better. The D2A in the dongle or Lightning headphones were likely not any better.
Unfortunately I do frequently with my laptop. I have had multiple employers lock out Bluetooth on work machines for security purposes, but I still have to take online video training. I typically leave an old wired pair in my desk.
Except for Lightning and the 30pin, I don't remember any Apple connections to be client/host off the top of my head. I guess the 3.5mm connection into the Shuffles, but that's a very odd case. Firewire was the same connector on both ends. I think ADC was too? It's been awhile since I saw one. MiniDisplayPort can be the same on both ends. Most of the other Apple developed ones I remember were ADB or AppleTalk. Still, I wouldn't call host/client a universal thing.
Maybe if it were a 100% swap of ports as Apple did when going to USB-C. That would not have been a necessity. Example: the current Mac Pro has both USB-A and USB-C connections. There would have been things to gain by letting Macs and iPhone/iPads share accessories, however. And again, they all have room for a 3.5mm jack which is bigger. The "sidedness" is also an issue on the USB-A connectors.
Edit: I forgot a counter case to the client/host in Lighting: the first generation Apple Pencil plugged into the iPad Lightning port which serves as the host.