I used Siri all the time to skip ahead while listening to podcasts, so adjusting volume might not be the stretch I initially thought it was. That said, if I need to adjust volume without asking Siri to do it for me, there's always the digital crown on my Watch, and there are also the volume buttons right on my iPhone. No big deal.
Other bluetooth headsets I've tried either don't have built-in volume buttons, or are hard to locate by feel (Jaybirds).
Perhaps a future version of AirPods could add a Magic Mouse trackpad-like sensor to detect sliding one's finger up or down to adjust volume. There's probably scant room for additional components more given all the other compromises to balance: battery space, antenna performance, compactness, etc. Another thought is a software-based solution that normalizes volume and EQ based on source material and ambient noise picked up from the microphones.
I am impressed that the most important function for me on earbuds, play / pause, is solved very simply by removing one earbud. When I am out walking, that will be much simpler to do when I run into a friend or neighbor and wish to chat, vs. my current routine of reaching and finding the inline pause button, pressing it, then taking an earbud out.
