Penn State never left. Sure, they weren't in the top ten for several years before last season, but given the horrific, unjust circumstances they were put through (that no other ever has), they did much more than merely survive. From 2012 to 2015, Penn State had a better record than a number of reputable programs. Penn State won a bowl game and played competitively in another bowl (against Georgia). Few, if any, programs could have done as well under the circumstances. Some likely would have shut down. All the while, recruiting improved year on year. All this with three coaching changes post-Joe (counting Bradley as one; I could also count LJ and make it four).
Going back a few more years, Penn State and Wisconsin are the only b1g schools with consecutive winning seasons since 2005. When you consider the sanctions that were intended to destroy the program, the fact that Joe was slowing down in his last 10 or so years, and that Penn State is still one of the few power programs to openly focus on academics and rules compliance, it's a wonder that they have been able to have such success.
Once and for all, everyone just stop with the "we are back" mantra. It's insulting.