There is no desktop "extinction" coming anytime soon.
That´s the same mistake as the claim that music CD sales going down is due to piracy.

The reality is that laptops and all kinds of similar options have been added, those who need only something less than a desktop now CAN get something less than a desktop.
Similar to how the spending on entertainment in total has gone up, just with a bit less going to music, purchases of computers have simply been spread out more. And with smartphones added, those also draws part of the purchases that used to pay for desktops.

AIO systems are hard to repair?

That'a a polite expression for "when one part of the system breaks out of warranty, you have to trash the whole thing."

This is exactly the same as for a notebook.

And let's not forget that AIOs are made with notebook parts.

Recently, I had to replace a dead monitor on my parents' Dell system. I went out and bought a new one, while keeping the PC box itself. This would not have been possible with an AIO.