@dobey It's a question of convenience:
Sailfish offers a very good overview over open apps. They even introduced a so called cover page with a specific way to display the essential content and to provide essential functions by buttons. Neither the stack of open apps in Android nor the fan arrangement is optimal for the overview and the interaction.
Further more the sidebar inherited from Ubuntu is not an otimal user experience. It inherits an approach form desktop Ubuntu. The approach is app centric, but it should be feature centric.
Another detail is the concept of a scrollable tab line. It is used in the system settings overview, that one can swipe in from the top and in many apps. It clearly shows, that this is not optimized for a mobile use case, because feature are not immediately available.
Sailfish is a really good example how a mobile user experience is improved. The font size is larger and functions are vailable with a very special pull down or push up menues from the top or bottom. Even if I define a large system font, the font size in the message app and dekko2 is very, very small.
Last but not least it's a matter of aesthetics. There is still a lot to do here in UBPorts. The introduction of themes, such as Suru Dark, is a big step forward.
According Superbook and Mirabook. I agree, there are missing hardware for a convenient convergence use case, yet. I'm pretty sure, there is a market for this use case an related hardware.