> 2. The iPhone becomes the new ultra portable desktop computing device. You plug it into a dock (maybe backside of an Apple 27" monitor) and boot up an ARM version of macOS from your iPhone.
Why would they do that - Apple is very profit oriented and if they can sell you Apple monitor, Apple iMac, Apple iPhone and all the dongles to connect them together, that maximizes their profit.
Plugin your iPhone into a monitor is not a good way to extract maximum profit - well maybe if it'll only work with Apple monitor and nothing else.
Apple does not work in a vacuum: PCs still exist, Android still exists, and things like ChromeOS are competing in certain spaces. Apple is also different from most competitors because they also make money after the device purchase from purchases of app, music, video, ebook, etc.
If the question was just “iPhone or iPhone + MacBook?” there's no doubt they'd prefer the latter but if the question is instead “iPhone or iPhone + Windows PC” I have no doubt they'd throw the Mac under the proverbial bus if it would keep your money from going to a competitor.
Why would they do that - Apple is very profit oriented and if they can sell you Apple monitor, Apple iMac, Apple iPhone and all the dongles to connect them together, that maximizes their profit.
Plugin your iPhone into a monitor is not a good way to extract maximum profit - well maybe if it'll only work with Apple monitor and nothing else.