As I pointed out elsewhere, in AmigaOS you actually did often compose the terminal/console handler with other software... The reason we don't in Unix-y systems is that we've gotten used to terminals implemented as standalone applications rather than as reusable components that integrate well with other things.
In AmigaOS it was fairly common for applications to reuse the standard console handler - the same one used for the shell - to provide a custom console or even editor windows etc.. For minimal integration all it takes is to read from/write to a filehandle.
That said, though as I noted elsewhere, even AmigaOS got scrollback in the console handler by '87 - the extra code measures a few KB; it'd be more hassle than it was worth to split it out.
In AmigaOS it was fairly common for applications to reuse the standard console handler - the same one used for the shell - to provide a custom console or even editor windows etc.. For minimal integration all it takes is to read from/write to a filehandle.
That said, though as I noted elsewhere, even AmigaOS got scrollback in the console handler by '87 - the extra code measures a few KB; it'd be more hassle than it was worth to split it out.