Something like this would be the smartest thing they could do, which is why I don't expect them to do anything like this. They could come up with a very flexible, easily upgradable, no monitor system that covered from Mini users to Pro users within the same hardware platform and get rid of Mini, iMac and Pro lines. I've never liked all-in-one systems since the first Sun all-in-one I purchased back in the 1990s. If the Mini hadn't taken a nose dive in the last release my plan was to replace my iMac with a high-end Mini and a 3rd party high-end monitor or two.
NOPE.
None of us know, but it's pretty certain that the All in One iMac lineage will continue.
I guess the problem Apple have with the iMac is that the screen is that good and the computing is that powerful - Enough for most mid-tier Pros (e.g. like me, a photographer) which is enough to cannibalise Mac Pro Sales.
The iMac is not good for Mac Pro sales.
This is what I think we'll start to see happening:
The iMac will go the same way as the MBP. Leaner, simpler, less sheer computing power and crippled RAM, meaning a 16GB MAX with no expansion). Thinner, no chin, USB-C all the way. Slightly bumped up MHz - more expensive. Wider Colour gamut, Faster SSD, possibly even touch ID/New Magic Keyboard. This will still be a capable machine, but just capped short of a pro spec.
Then, if you want more you will have to buy a *New Mac Pro - Increased performance, ultimately expandable, Superduper graphics cards, external Thunderbolt 3 yadda yadda yadda. It will again cost more, but that will be your pro machine.
The iMac started out as an "Internet Computer", became a very powerful desktop solution, and is now plateauing. It might be a good time for something new - I wouldn't be surprised if we see a whole move into proper cloud computing from Apple to take some of the load from the actual computer, for example. iCloud is so very near being a good and well implemented idea, but without it evolving into something useful, instant, collaborative, and accessible and immediate, I'm afraid all Apple will achieve is providing lower specified computers to their loyal customers while asking for more money for doing so.