You can generally get replacement centre console surrounds that add DIN or Double DIN holes to cars that didn't come with it. I've done it to a few cars for friends. Search eBay for your car make/model + "double din". It's not available for every car, but it's available for a lot.

You can also 3D print your own. There's free downloadable models of the DIN slots that you can add to a 3D scan of your cars centre console. Then depending on your car's interior trim, fibreglass it, or trim it in leather/whatever you want. Or you can just buy the DIN/Double DIN housing, dremel out where your stock headunit is embedded, and mould it in. Plenty of people on car forums do this and there's lots of guides online.

Considering the absolutely insane premium prices for sub-par entertainment systems in modern cars, I've personally found the above effort completely worth it.

I'm currently adding a RasPi and 7" touch screen with GPS for Google Maps, 1TB SSD of media, Spotify (via Wifi tether to my phone), USB ports in glovebox for USB sticks of media or charging phones, and more, to a 2012 car that came with an embedded CD/MP3 player with no nav, no screen, and no DIN slot.

True but now they can offer their own systems that integrate far better with the car electronics. Such as controlling the HVAC or offering rear view camera in the in-dash system. Those are good opportunities for lock-in which they'll no doubt take advantage of.