The 'old guard' of Mahler scholars are slipping away from us. One wonders if a newer generation will make anywhere near as interesting or informative contributions. That said, however, it seems to me that the most interesting commentaries can found in CD liner notes by people I've never heard of. This is sort of typical of our democratic, 'faster information' age. In short, they don't need a contract to write a book in the hopes of getting their views expressed.
Anyway, Donald Mitchell certainly made some interesting and - at times - highly speculative commentaries.