The switch to PG was never going to be permenant, everyone knew that. The reason they did it, i presume, was to rebuild their fanbase by luring in younger audiences since wrestling as a whole lost much of their audience to disinterest, MMA, etc.
I got into wrestling when i was 8, just when the Monday Night Wars was heating up. Wrestling got more and more exciting and mature from there which kept my interest going and cemented me as a long-term fan. Minus the Monday Night Wars, WWE is going through that period again: Now that PG-13 is creeping back, the younger fans are going to enjoy the product more as things get a bit more mature (and they get older along with it).
PG wasn't a "failure" - it was a period of WWE rebuilding themselves.