Yeah, I think it will affect his legacy if he doesn't return especially if he never wrestles again period. Then again, even if he did wrestle for another company, there not many mainstream promotions that would give him the opportunity to be on TV like WWE did other then TNA, which I highly doubt he would end up at.
Imo, he went about the WWE thing the wrong way from a professional standpoint. When he walked out, it wasn't like he had something major on the back burner like having a role in a major movie or something. Other then a few appearances here and there at stuff like UFC events and The Walking Dead TV show, he's basically been doing next to nothing to keep his star power hot.
Now, I know his WWE contact prevents him from doing certain things, but if he really wanted to be done with WWE, why not just honor your contact, wait for it to expire, and move on to something else. Instead, he's burning bridges and potentially hurting his legacy. Even if he didn't want to do anything else but just retire and get married, if he had just honored his contact, his legacy wouldn't even be in question.
Instead, now the WWE is doing petty shit like Photoshopping him out of images. I wouldn't even be surprised if the WWE decided in the near future to have someone move ahead of him as one of the longest reigning WWE champions just as a "fuck you".
But hey, maybe Punk just don't give a fuck. He's still rich and he's smashing AJ. I can't feel sorry for him.