Maybe if Robert Schumann’s symphonies were to be played on “period” instruments, Mahler might not have found it necessary to rescore them. Or maybe Schumann’s scoring is so bad they need work done on them anyway.
Mahler’s rescoring of the Schumann symphonies in order to make them more transparent had become necessary because of the development of the size of the orchestras as well as that of some of the instruments.
As becomes now clear with the use of period instruments and orchestral sizes, Schumann did not orchestrate his works so badly at all. Some of the now rather greasy sounding doublings, especially in the winds, do sound transparent on “period” instruments.
The opening of the Symphony No. 1 played on valve horns at the original pitch, i.e. a third lower than normally heard (as natural ones cannot cope with this, the original scoring), makes a real difference. The horns which define so much of the festive character of the Symphony No. 3 sound really exciting, and the clarinets in both the original as the revised versions of Symphony No. 4 do colour the piece – as a more protruding solo violin does as well.
So, Schumann’s orchestration sounds “fatty” more because of being played on instruments for which it basically wasn’t meant, than because of Schumann’s supposed lack of experience/knowledge of orchestration.
But it depends which of Schumann’s orchestral works: there is a marked difference in the orchestration of those from after 1850 (including the Third Symphony and the 1851 version of the Fourth), with much more doubling of string parts in the wind, and cellos and basses generally playing together most of the time, than in those works from the 1840s. It was at this point that Schumann moved to Düsseldorf and took charge of the orchestra there, which was somewhat smaller than the Leipzig Gewandhaus which had earlier been his basic model (the Düsseldorf orchestra seems to have had about 50 players, at least in 1852, though these were occasionally augmented for big festivals; the Leipzig orchestra had 60). Also the string players were of a markedly lower standard, as attested by Wilhelm von Wasielewski; Schumann brought him over from Leipzig to be leader of the Düsseldorf orchestra. My interpretation is that Schumann’s late orchestration is pragmatic, designed to get the best results out of the forces he had available on a regular basis at that time. With that in mind, when using larger and better orchestra, I do believe there may be a case for considering Mahler’s modifications, or those of Felix Weingartner. However, it should also be borne in mind that Schumann advocated, in a letter to Franz Brendel in 1847, a section of a Universal German Society of Musicians for “the protection of classical music against modern adaptations”, and to research “corrupted passages in classical works”, about which he had published an article in the Neue Zeitschrift für Musik.