The way I think about adapters is that you have a Lightning cable and attach the adapter so it turns into an USB-C cable. I guess you can also think about adapting the port. Wild.
Who is this for? Looking at the Apple Store, they sell an official USB-C cable for less ($19, but you can obviously find good quality third-party cables for much less).
People who have accessories with a lightning adapter probably. Speakers, docks, etc.
Are those common? I don't think I've ever even seen a speaker with a built-in Lightning dock.
I remember these iPod docks were very common at the time of the 30-pin connector. However by the time Lightning came around wireless audio became more commonplace with Bluetooth, AirPlay and other related technologies (CarPlay, smart speakers etc.)
Not in the last couple of years, but 11 years ago when it was introduced, probably.
Airplay was already around when the 30 pin connecter was still there, and they were also popular back then. I remember using a 30 pin connecter speaker with a 30 pin to lightning connecter also because it could charge my phone at the same time.
Realistically, the market for such accessories is likely fairly limited, but that in the end also results in a higher price (no scale in the market after all).
And they can use their current phones with them, no? No one is forced to upgrade - they can easily buy another lightning-having iPhone when their current one croaks and should be safe for the next 20 years (buying used iPhones) if their stupid accessories are so gd important