Sometimes it's even more obvious. Consider the beginning of the first movement of Schubert's D 958 quoting Beethoven's WoO 80 variations.
Many other famous examples. Pathetique quoting K 457, Beethoven C Minor concerto quoting Mozart C Minor Concerto, Fantasy Impromptu quoting Moonlight Sonata Mvt. 3. And even more examples if you look at baroque music! Of course, you can always see them in sets of variations. What makes it "outrageous" that composers are respecting other composers' works? Of course, there may be a day when all the good chord progressions are taken. Beethoven is simply using a good theme Pachelbel created.
(I can't ever help necroposting. I don't know why.)