I've got Termux running on my phone, complete with vim plugins, language server support, several compilers and all kinds of other tools. Combined with a bluetooth keyboard, it can be very useful in a pinch. It'll stop working on Android 11 because of "security concerns", but either thankfully or sadly, my phone has no stable Android 11 release yet. Everything is running inside a sandbox, I don't even have root access, and the binaries are distributed through a normal Linux package manager. With the right software you can even run a normal GUI on it through VNC or Spice, although that's something I haven't explored yet.
No need for other app developers to have any relation with Termux, that's what the sandbox is for. On Android, you can theoretically implement a system for sharing binaries and virtual files quite easily if Termux would support it, but I haven't seen such need myself.
These tools are maintained by volunteers and the Termux developer, and can be extended by adding repositories made by other people. So "who is going to maintain and distribute all these parts" comes down to the same question as "who is maintaining and distributing all of these Debian packages": the developers who want to make the ecosystem and apps function.
Most users won't use their phone or tablet like this, but I honestly don't see why they shouldn't be allowed to if they wish to. Apple is selling a complete keyboard and display stand for iPads, so these devices are clearly being targeted for productive use. Yet Apple refuses to allow developers to be productive on these devices, because they don't want competition for their crappy mobile browser engine.
As far as hardware is concerned, the touch screen, keyboard and OS are pretty much the only serious differences between the iPad and the Macbook Air. If you prefer a two-in-one tablet/laptop combo (which quite a lot of people do), the iPad is the closest Apple product to fit the description, if it would allow users more software freedom.
I do see the advantage of the curated app store, but I don't see the advantage of banning customers from not using said app store for the end user. You don't _have_ to install any apps from outside the app store, you just get the option to do so if you wish. I don't know any non-technical people who have installed apps from outside the Play Store, so it's not like allowing any lifted restrictions will make the ecosystem collapse.
I have a hard time understanding why you would want a company to tell you what you can and cannot use a device for. Their suggestions are always welcome, but why would you be in favour of their restrictions?