Just had to add a minor comment. If you look at things as an evolutionary process, then you must realize that things become more specialized as the process goes on. Starting with titans leaves you a potent genetic base. Even if they were to relegate "Weaker" members of their society to become specialized and become different races, there would be some constant evidence of what had gone before. Of course you can look at the option of having the standard apocalypse that removes most of the evidence. There are other options....
Remove the genetic connection, or make it incredibly distant. The powerful titans were masters of magic, and transformed lesser beast into divergent races of servants. If not lesser beasts, perhaps some of the wars between factions ended with the transformation (a la polymorph) of losers into slave races. Removing them as a real physical threat, and placing them in specialized forms that ended up flourishing.
Rather then titans, it was simply an advanced hominid that was more adaptable then any other creature out there. However, once they became too entrenched in a lifestyle, the adaptability lessened, until the genetic factors weeded themselves out of the gene pool. Sort of a Proto-Doppleganger, that evolved itself into non-existence.
At any rate, just a couple thoughts.
