[npunt]

Agree with your second point.

Re:cannibalization, Apple is more than willing to cannibalize their products - just see iPod Nano v Mini, iPhone v iPod, iPad v Mac, iPad 2018 v iPad Pro. It's part of their playbook to cannibalize themselves, so that competitors don't wind up sneaking in market segments. This is enabled by organizing the company functionally rather than by product line, which lets them avoid the 'strategy tax' of pre-existing divisions that own products wanting to keep those products alive (bureaucracies self-perpetuate and all that).

That said, a phone that converts to a desktop (at least in how I think OP and others who bring this up think of it) is not part of the playbook, because its making the device do double-duty in UI/UX. See iPad not having a mouse.

I think Apple's perspective is that the glue that ties mobile UX to seated/desktop UX together is the cloud, and to your point, that involves multiple devices. The exception is non-interactive content (AirPlay), which third parties can license.