I don’t think “the conjunction” will happen. I believe Apple will keep two product lines for long enough to surprise you. One will continue the “desktop” workflows and macOS the other will continue the “handheld” workflows and iOS.
If there is a conjunction, it’ll happen in two places, which will seem like one place at the UX layer. The UIs will carry alike style and behaviours, so that you have a consistent experience of “the Apple Way” across your devices. The UIs will downplay the mechanical differences and share visual metaphors for what data looks like, how you interact with it, and where “computation appears to happen. Your “desktop” will look increasingly less like a desk top and more like a window into scintillating infinite void.
The other point of conjunction will be where the platforms hand off to iCloud, Siri and whatever follows them. There’ll be a seamless blend of local and remote compute, secured and anonymised as far as Apple can manage, and you wont notice as you move from situation to device. It will appear as if “the Apple Way” is woven through as much of your day to day life as Apple can get you to buy into.
And Google and Samsung will make bad copies of it that don’t work as well, or that work better but sell you out to advertisers.