Some people do.
I don't think so.
The only way they're using it is the same way they're using a Commodore 64, as an antique.
A 486 running Windows 3.1 with Cubase 1.0 exclusively for MIDI running in Standard Mode is far superior to any modern machine with respect to timing. Also, MIDI software hasn't evolved that much over the past couple decades.
Perhaps it even rivals an Atari ST
DOS/Windows 3.1 is still good for timing sensitive applications.
Nowadays MIDI interfaces are usually USB. But an ISA card or a parallel port will yield better results.
On modern computers, MIDI generally feels slightly off, and not as musical, but for the most part 'good enough' for the vast majority.
But the oldest iPad from 2010 will still be able to get the “last supported version” of apps from the App Store, still works with old versions of iWork, streaming video apps, Spotify (at least it did a couple of years ago), you can send video from it to any device that supports AirPlay and print to any AirPrint printer.
But to respond to your second point, I believe that Apple has REMOVED apps from the App Store that were previously available to download. For example, I was not able to download on my old iPhone 4 the latest working GarageBand for that device, because it was deleted. This is not the only app that I have not been able to download, because it was REMOVED FROM THE APP STORE.
But I think the point you are missing is that if Apple decides that these devices are obsolete and they will not support them, they should be willing to at least release documentation that allows people to write/install whatever software they want on those devices.