Ironically the only cars I can think of that this would be useful for do not have bluetooth, aux or usb either,
There are plenty of vehicles on the road that have bluetooth, aux, and/or usb that do not have CarPlay or something similar.
CarPlay launched 7 years ago, but it took a while for many car manufactures to get on board. Aux input has been a staple in most cars for almost 2 decades.
For myself, I own five cars:
Only one had CarPlay from the manufacture, 2019 Jetta.
The next newest one from 2015, a Mazda5, I installed the CarPlay head unit. That one, and came with bluetooth, Aux input, and USB input, but no infotainment system.
The next one, 2003 Mazda Protege5, I installed an non-CarPlay head unit.
The next one, 2002 Ram 1500, had a non-CarPlay aftermarket head unit installed when I bought it.
The oldest, 1991 SW20, I installed non-CarPlay aftermarket head unit.
So in my own personal experience, 80% of the vehicles that I currently own could have benefited from Spotify's Car Thing.
My case may not be the norm, for people are keeping their cars much longer.
Because, at least in the US, people are keeping their vehicles longer. Many vehicles do not have CarPlay nor any infotainment system.
The average age of vehicles is currently over 12 years old, and that number keeps climbing.
2010 is about what the average year of the vehicles on the road in the US, so you are not too far off.
According to this, the average vehicle in the US was 12.1 years old:
The average age of vehicles on U.S. roads has been rising for decades as cars grow ever more durable — and expensive.
www.cnbc.com