So there is no defense against Obliviation that Voldemort could have prepared for himself?

With perfect sight toward the future, perhaps he could have. It's far from convincing that it would have actually helped, without blocking thirty other vulnerabilities.

Obliviation's particularly interesting because it requires no upkeep, but it's far from the only thing that would bypass Horcruxes. Voldemort's just as vulnerable to being repeatedly stunned, to petrification (hence the murder of the basilisk), to transfiguration, to the Imperius, to pretty much any mind-affecting charm. The biggest defense is, well, the same as anyone else's defense to the Killing Curse -- don't be there. Creating a defense specifically against large-scale Obliviation isn't very valuable if the attacker has countless further options to permanently disable anyone so defenseless as to be vulnerable to an Obliviate.