With that being said, I don't see Apple reversing course within four months of WWDC and suddenly deciding to merge the iMac and iPad Pro lines, it just doesn't make sense from a market segmentation perspective.
They're not going to "merge" the lines - they've said as much. The Apple Silicon Macs will run MacOS and the primary distinction between an iPad and the Mac will be that the iPad can't run MacOS (although Apple could make that happen if they wanted...).
On the other hand, it's a known fact that Apple Silicon Macs
will be able to run native iOS Apps. That's a
huge library of applications in an age where - at least in the consumer sector - "available for iOS or Android" is the new "available for Windows".
What that also means is that there is now a really strong case for a "2-in-one" touchscreen Mac because, suddenly, there's a shedload of touch/Pencil-oriented software including some titles that will feel very clunky when used with a keyboard & pointer. The problem with PC 2-in-ones (which are nevertheless popular) is that there was never such a critical mass of touch-optimised software for Windows.
You wouldn't detach the screen and stick it in your briefcase to use on the road - you take it off, maybe flip-out a kickstand and use it in "easel" mode when working with touch-centric software. Maybe even hook up a second screen - that would be great for, say Logic Pro, which already supports using an iPad that way. Then put it back on the stand when you need to edit a lot of text or numbers (which touchscreens are
useless for).
The other part of the OP post that doesn't make sense is that the new iMac will be thin because Apple will "no longer need HDDs, nor video accelerators, nor fans", as they will absolutely have to keep pace with Intel and AMDs integrated graphic solutions and preferably blow them out of the water if they want to make Apple Silicon relevant in the desktop/notebook market.
You're assuming that there is going to be "
the new iMac" (along with
the new MacBook Aid and
the new 13" MBP).
As I've said previously, there are
already effectively four different iMacs. - 21.5" HD, 21.5" 4k, 5k, iMac Pro - that cover a range of applications from relatively cheap consumer/education to fairly serious pro. To consolidate those into a
single model would be rather "courageous".
The entry level 21.5" (with integrated graphics) could comfortably be replaced by a passively-cooled A12Z,let alone the first-gen Apple Silicon. The higher-end 5k iMac/iMac Pro that come with fairly serious GPUs may have to wait for 2nd gen Apple Silicon with extra GPU cores and a different balance of power & economy CPU cores.
So - although nobody knows what Apple will actually do - a ~20" 'convertible' model that could be used as a "desktop tablet" (with iPad-like design language) at the lower end, and a more conventional 27" (or larger) all-in-one to replace the 5k iMac/iMac Pro (maybe with Mac Pro "steampunk" design language) isn't an unreasonable concept.
Samsung actually tried this with their Galaxy View, which has an 18.4" screen.
Samsung and Android in general don't seem to have really penetrated into the "pro tablet" market, though (and even MS Surface seems to be mainly used as a laptop running full Windows apps). It's an area where Apple have been expending a lot of effort, making the most of iPad/Mac interoperability and encouraging people like Adobe to make "serious" iPad versions of their applications, so there is already a body of iPad software - particularly for freehand graphics and music - that could take advantage of a larger tablet format (esp. if it was an iOS/MacOS 'convertible').