I think you raise a good point. What's interesting is that Mahler doesn't proportion just long he wants the vowel - "e" - sustained, and how much time he wants spent on the "sch" part. He simply writes a whole note for the entire word, "Mensch". Given that fact, I would think that singers would want to spend as much time as they get away with on the "e" part (eeeeeeeeee), and very little on the "sch" business. But it's almost as though some singers relish that "sch" at the end. In some cases, it really becomes an affectation.