I have to disagree with some of Barry's assumptions.
Who would have conducted the 9th and Das Lied in 1911 and 1912 if Walter didn't? And why was he the conductor and not Klemperer or Fried or Mengelberg? Who would have recorded them in the late 30s? Who else performed Mahler in the US? Mengelberg wasn't allowed to conduct much Mahler in his later career, alhtough he was a great advocate during and for a while after Mahler's life. Did he perform Mahler in New York? Fried did have the acoustic Mahler 2, but that was limited and an old fashioned format by 1926.
I know Barry is a bit divided over Walter's performances, but Walter was still the major international conductor performing Mahler and publicly promoting Mahler for many years. And his recordings of 9, Das Lied, 4, and 5, all before 1950 made innumerable converts to Mahler. My first Das Lied was the 78 set borrowed from the Phoenix public library in 1958 with the Mahler profile labels. I couldn't afford the London LPs.
Just having great art lying around is not sufficient for it to become known. Think of Bach's choral works before Mendelssohn become their advocate. Or the Janacek opera before Mackerras performed and recorded them.
Others made important contributions, of course, but I think it historically inaccurate to underrate Walter's major role in enlarging Mahler's reputation over four decades.
Best to all,
Tom in Vermont