FWIW, I think the external display support in iOS 12 may be related to the fact that switching to USB-C means that Apple's display adapter no longer works. That display adapter was essentially a tiny computer, so I would imagine there was a lot of logic they could shunt off onto that adapter and not have to implement in iOS, especially since it seemed like they were very resource constrained in terms of iOS engineering for a long time. Now that USB-C obsoletes that display adapter and also natively supports video, they may just be in a position where it makes more sense to do all the work in iOS rather than develop another hardware adapter.

Of course, the optimistic side of me takes this plus Marzipan and comes to the conclusion that they are putting all the pieces in place for iOS to fully support laptop and desktop form factors and work flows.

Fingers crossed.