It is certainly true that Dohnanyi was not deserving of the neglect of so much of his fine music.
It is not wholly correct to identify the source of this neglect in his being out of synch with the times.
If this were true, then we could not account for the great popularity of Rachmaninoff, Kodaly, Respighi, Khachaturian and many other well-known composers who wrote in a traditional vein.
One has to admit that Dohnanyi published several composition of a low level of inspiration, which weigh all the more heavily in an oeuvre of only 48 works.
One of his trademarks was too great a reliance on variation form. There is nothing intrinsically wrong with this, but if it occurs in one work after another, musicians will sooner or later wonder if it serves as a substitute for original invention. The other of his problems was that he sometimes began ambitious work with powerful ideas, but could not sustain them.
His Concerto No. 1 is one of the latter. It is a great lumbering affair, notable primarily for its enormous length and the fact that after a promising beginning, he ran out of ideas in the middle of it and resorts to much padding and stitching together of ill-fitting subsidiary episodes. If you listen without attending, it can sound very pleasant, with melodies of great sumptuousness surfacing from a rather nondescript "working out". But one could hardly take it seriously as a work of art. As such, it falls apart into disjunctive episodes and ultimately the logic of concerto writing goes begging.
No. 2 is a different kettle of fish. Dohnanyi learnt his own lesson and presents his material with great concision. The solo part is well integrated with the orchestral canvas, the material draws attention to itself by its superb individuation, and the whole things sweeps past you as a well-constructed whole.
It is undoubtedly a masterpiece of its genre, the neglect explicable only by the fact that Rachmaninoff, whose music seems cut from much the same cloth, achieved much greater success and put Dohnanyi in the shade.
This recording gives a fine advocacy of both works. I have heard a more incisive reading of No. 2 from (I think) Peter Katin, decades ago, which unfortunately I don't have any more. It was done at a time when Dohnanyi was as dead as a doornail in public estimation and filled up a disk devoted to the Variations on a Nursery Rhyme, which I think damaged Dohnanyi's reputation more than he deserved.
But these performances by Roscoe will serve adequately to keep his memory alive as a great pianist and orchestrator.