UFC is not a realistic career option for a man in his late 30's. He made a lot of cash in his debut match and that should be enough of an accomplishment. Nothing embarrassing about it.
He should go back to wrestling. He obviously won't return to WWE on his own. He is a man with too much pride.
Vince is a smart business man and doesn't let his ego get in the way. He has brought back people that burned bridges with him in the past. If it's good for business, and Vince bends over to Punk to bring him back and give him the main event in WM as promised, there's a good chance Punk might consider his offer.
Do you see Punk going back to the WWE in the near future or just retire?