Yes, a very good point.
The OPs question was about “when does the Mass end?” not “when is it polite to leave?”
The question itself might seem insignificant or merely academic, but it points to a larger issue.
Contemporary “liturgists” (I cringe even typing the word) have made it a regular practice to confuse issues by switching such questions to justify their own personal changes to the Mass.
How many times do we hear “No, it’s not in the rubrics, but it’s still right or necessary to add it to the Mass” ??
Why do we ask visitors to stand up before the penitential rite? “because we must.”
Why do we repeat the alleluia after the gospel? “because we must”
Why do we change the words of the Lamb of God? “because we must”
Just a few typical examples to make the point.
Now, the question here is NOT about making changes to the Mass; however it is about liturgists changing the questions to suit their own priorities.
When liturgists answer the question of “when it it polite to leave?” by saying “the Mass is over [when]” they are confusing two different issues. They know that they are in a much stronger position when they can say “you’re leaving before the Mass is over” instead of saying “it’s impolite to leave before the hymn is finished.”