Later on in deployment, it will go somewhere else. Somewhere that has been evaluated for being able to handle it.
In that way, /dev/null is to storage what `true` is to execution - it just works.
Both (along with a lot of the standard utilities) are a testament to what talented C programmers plus years of people beating on them in unintended ways can achieve in terms of reliability/stability.
The newline is a line terminator, a command outputting an incomplete line without a line terminator is producing garbled non-textual output. Files which contain incomplete lines without a line terminator are similarly garbled non-textual files and not very different from /dev/urandom or any other binary file.
I would argue that no, there are many valid cases for commands to not produce a final \n in their output. The first example that come to mind is curl'ing a REST API whose body is a single line of JSON. Many of those will not bother with a final \n, and this does not qualify as "garbled output" in my book. I would even go as far as saying that a shell just printing the prompt at whatever place the cursor happens to be is a side-effect of how terminal emulation works and the fact it's just a character based terminal.
This is actually something that Warp does pretty well, with a strong integration with the shell where your command is in a dedicated text box, by the virtue of it being GUI and leveraging GUIs. (I don't use it however, I'm too much of a sucker for dense UIs).
However I do agree with your argument that it's not the role of the shell to protect you against `cat /dev/urandom` or `cat picture.png`. And fish indeed does not try.
IMHO a shell is built for humans when in interactive mode (one of the raison d'être of fish), and the lack of final \n is such an annoyance that handling this specific edge case is worth it.