Well, that'd be an argument against ghost encounters, not necessarily against "any kind of afterlife". There's plenty of afterlife beliefs that don't involve ghosts at all.

Moreover it's assuming that being a ghost would actually impart some incredible insights, or that the behaviour or experience of being a ghost would necessarily reflect that. Most of what I know about ghosts comes from the movie Casper (1995), and in that ghosts are depicted as beings who can't move on from some trauma from their life. Maybe the incredible insights come later, after moving on; maybe the nature of being a ghost necessitates fixation on the "mundane".

I'm also not sure (nor am I not not sure) that most accounts of interaction with ghosts would even be described as "mumbling on about something mundane". A Christmas Carol (fictional) depicts ghosts as providing life-changing spiritual guidance, for instance.

The best - and only - argument we have against belief in any kind of afterlife is that we have no actual evidence for one. The very idea lies outside of our capacity to know.

Well, you don't know what you've got 'till it's gone, right? Maybe the first thing we start missing is small talk. (Of course, I'm more of a Lisp guy myself)

Of course there are also stories of people who claim to have left their body and seen the operating theatre from above and to have noticed some unexpected object on top of a cupboard or in some other location visible only from on high ... and later, when someone checks, the object really is there ... [spooky music] ...