A couple of thoughts.
One, there are regional differences in how the concluding hymn is perceived. In some areas of the country, the song is intended to accompany the priest and the congregation as they leave. The expectation is that the priest leads everyone out. In fact, this is the way it was always done when I was in Catholic school. It would be unusual, in one of those parishes, to not leave while the last hymn is being sung. I have even seen parishes where everyone goes out while singing and then many come back in to say private prayers.
Two, there are differences in the kind of music that is chosen for the last hymn. Sometimes it is a congregational hymn and it would seem more natural to stand and sing together. Sometimes it is more of a choir piece in which case there is no real reason to stay. In fact staying makes it look like the choir is performing.
Sadly, there is another reason, which happens sometimes in my own parish where the last hymn is just not worth singing. I would rather move on out to the community gathering than to stand there silently until the musicians stop playing.
As long as the people leaving aren’t doing so while carrying on loud conversations, I don’t see a problem. The Mass is already over at that point so there are no “rules” as to what we are supposed to do.