The players let it happen, so no, it's not not their fault. They could have called shenanigans, as could you. Frankly, I'd talk with the players, including the other GM, and find out if this is what they want to keep playing.
Explain your concerns, don't play it up as the old GM's fault, any one of you could have said something. Regardless of why no one called either of you on the power level, it's where you are now and you're all responsible for being there.
If most of the group wants to keep playing at this power level and you don't want to GM, let the other guy take over and either stick to playing or find a new group if you don't like being that powerful yourself.
Otherwise, start a new game and make it clear that you're not hot-swapping GMs. If the other guy wants to run for a while and you want to take a break, keep the games separate, build new characters for his game and leave yours on "pause" for the duration. Heck, those occasions where you're feeling a little burnt out and want a month or two break are great times to try out different systems, too. When I was getting bored with my DnD game, one of the players GMed an oWOD Vampire game for a while, fun and hijinks ensued and the other game was better for my relaxation and didn't have the confusion of two different GMing styles.
FastJack's solution works well if you don't feel you can have an honest conversation with your players, or they're too attached to the characters to walk away but don't mind a drop in power level.