I'm going to assume that a history of computing class would talk about the useful age of any architecture.
When you see film of older systems, it looks like a fun, or cool, or "retro".
But in our daily lives, there are two architectures that have been around for a long time: macOS and iOS.
So when people have been gossiping about various ways Apple can merge the two, I ignored it. But now that we've reached the rumor phase, and the proposals are more specific, I can start to think about it.
Here's where I'm concerned: hardware integration.
I'm not talking about screen size or aspect ratio, I'm talking about everything.
Take a look at the pictures in this "Ask Different" post:
The original feature was: let the app dynamically update the application icon (in the dock, or in the "Scroll through all open apps" view) with the CPU status (either Windows-style activity thermometers or the historical load).
It worked great for one or two CPUs. It got confused when hyper-threading made each CPU appear are 2 CPU's in the display.
But on any "8-CPU" system, it looks like a bunch of vertical bars.
Apple's docs on app switching