Beethoven carried the process of motivic development within the structure of the sonata form as far as it could be carried. He stretched and rearranged the sonata form quite a lot, but it remained the conceptual framework for all his work. Wagner divorced the idea of motivic development entirely from the sonata form, from virtually any form unrelated to plot, and expanded it to gigantic proportions. He forms the logical conclusion of the idea, making him the culmination of the past and not at all the music of the future that he imagined.
If you don’t mind all that constant modulating, Wagner is wonderful. Except for Parsifal, of course. His use of the borrowed theme of the Dresden Amen which he repeats ad nauseum, always in its original borrowed harmonization, cannot be considered thematic development in any sense. It’s a mistake, a serious lapse in judgment, maybe even a sign of senility. The emperor has no clothes.
I apologize for telling such a sad story.