Wagner is atypical of the operas produced in his time. There were some composers who did Wagner rehash after, but they are small fry now. The big impacts where by guys who took Wagner's ideas on board in opera and did their own thing with them, not just glorified rehash. Two biggies there are R. Strauss and Berg. But their things came after 1900, in the case of Berg's Wozzeck, in the 1920's
Wagner's impact was immense... and in no way limited to opera. Yet I agree that he had no clear immediate heirs in the field of opera. It is later composers such as Richard Strauss, Debussy, and in his own way, Puccini, who took the ideas of Wagner further... in their own unique manner. Of course this might be said of most major influential figures. Beethoven has no immediate clear heir (although some might suggest Schubert... who was really but a younger peer). Brahms is the most obvious composer who builds upon Beethoven... but Brahms is later and like Strauss and Mahler, has his own unique voice and vision.
So both Mahler and Wagner revolutionised their genres, but they were pretty unique in what they did. By the time their innovations and refinements became more kind of mainstream or understood (well, by composers at least), the whole ball game had changed. The era of mammoth things was over, basically... the era of bigger being better was larely over, seen as an anachronism...
I don't know if I would agree with this. While later composers may not have rivaled Wagner's operas in terms of scale, you do recognize a similar richness and grandeur of orchestration in Tchaikovsky, Mahler, Debussy, Ravel, Rimsky-Korsakov, Rachmaninoff, Korngold, Zemlinsky, Szymanowski, and even Stravinsky. I agree that there was a certain intimacy of scale... born of chamber music and the cabaret as well as (in part) an intentional rejection of the extremes of Romanticism. One sees the same thing in early Modernism... in the small cubist paintings of Picasso and Braque, and the initial abstractions of Mondrian, Kandinsky, and Klee... but Modernism soon reveals an audacity and grandeur of intention equal to Romanticism. Richard Strauss' works are no less rich and lush than those of Wagner. Benjamin Britten's War Requiem and a good many of his operas are no less ambitious. Nor are Shostakovitch' operas and a number of his symphonies, or Prokofiev's ballets. Taken into the late 20th century we find a certain irony involved in the fact that the composer often seen as the epitome of Minimalism, Philip Glass, has produced some of the longest and least "minimal" operas. Scale and ambition seems to be something that continually wavers in the arts. Today, the visual arts are dominated by a grandeur of scale which in many ways in contrasted by the vapid and shallow vision. It is as if all this bluster was but a bluff for lacking anything of substance to say. This most certainly was not true of Wagner and the greatest of the Romantics. In a contrary manner, we should recognize that the intimacy and diminutiveness of works by Satie, Debussy, Stravinsky, Shostakovitch (the Preludes and the Quartets), Berg, etc... should not be confused with a lack of ambition.