Regarding the switch from Raspberry Pi to another small computer:  the ready made images for Raspberry Pi are not compatible with other computers.  But most of the programs are.

The question is about the OS: Armbian  is just a normal Debian compiled for different ARM systems (including Raspberry Pi) and it offers almost the same as "normal" Debian.  It is different than Raspbian, which can run only on Raspberry Pi.

There are some differences - one of them is for example interface to configure WiFi ...

Unfortunately not much to add. I think there was an assumption that Raspberry Pi's would always be available and it's turning out that it's unfortunately not the case. I'm guilty of this for standard chips. I didn't think there'd be such a huge shortage that spread across so many chip families which left us scrambling.  I'm sure the supply crunch will ease up soon so the RasPi shortage should ease. It should be considered thought that should something happen to the Raspberry Pi Foundation, there's a possibility that the production could end. Since Raspberry Pi's are not open source, this would be devastating to many projects that rely on them. 

As for porting to another platform, I guess the question would be which platform to bet on for the Raspberry Pi alternate and possibly eventual successor. It would probably need to be open source, widely available, and have enough community around it to support itself. Nothing immediately comes to mind (Jetson possibly but not open source and subject to whims of nVidia).

And perhaps this could turn into a community project to come up with an open platform that conservation communities can standardize on where availability could also be guaranteed. There's a lot of exciting things happening with the RISC-V architecture, which could be a good candiate.

Yes, but for our needs that are not powerful enough