The popup autocompletion can sometimes be obtrusive, distracting and slow down typing to the point that some users just prefer to disable it.
Inline auto completion would be an optional auto completion preference. It would show inline, the result of pressing the tab key (or whatever shortcut the user has configured).
Inline and popup autocompletion modes would not be mutually exclusive. Both could be enabled at the same time, or just one of them.
Here is an example of what I'm talking about (from Fish Shell)
Customer support service by UserEcho