"Never say never" as the old saying goes in professional wrestling.
Remember when the news broke that Kurt Angle was going to TNA? I was a lot younger at the time, but even I knew enough then to know that was a huge deal. Obviously, there's a difference between Angle - who had medical and drug issues - and CM Punk, who is straight edge and, baring a few recent injuries, in the peak of his physical ability.
Jeff Hardy going over a few years ago wasn't a huge surprise because of the drug arrest, and having already been in the company before. Rob Van Dam wasn't ever "top tier" enough to shake things up. It's been so long since Sting showed up, but I'm guessing that was a pretty big deal to TNA fans at the time.
The closest thing to Punk crossing the line is Hulk Hogan signing a deal with Dixie Carter. I honestly thought it was a joke when it happened. Biggest name in the history of the business, coming to the 2nd rate company that everyone hates on? Crazy. But again, he wasn't signing on as an active wrestler. However, the fact that I can still remember that January 4th, 2010 was a big deal is in fact, a big deal.
CM Punk has always said he gets bored easily and is looking for the next mountain to climb. He's also said multiple times that he does't want to be a "lifer" in pro wrestling. That he probably won't stay with WWE for that much longer. Of course "that much longer" is always a relative term. Could be two years, or ten. But what's the mountain to climb in TNA? What's Punk's motivation for likely taking a giant pay cut, and going to a smaller show? Making a difference perhaps? Out of everyone that's a big name in WWE, Punk easily seems like the guy that would duck out to cross that line.
Look...here's the thing about all the wrestling fans saying "this thread is ******ed; Punk will never go to TNA!" Let me take you back to 1993, the year Hulk Hogan left the WWF. He spent a year hanging out with his family, working on terrible movies, working in Japan, and doing other things to pass the time. And then...in 1994, he did the unthinkable; Hulk Hogan signed with WCW. So did Kevin Nash and Scott Hall. But more importantly, HULK HOGAN SIGNED WITH WCW. The secondary company that was nowhere near the popularity of the big, WrestleMania-driven engine. The company that caught a lot of crap for the people that ran it, and the networks it was on, and for some of the talents they pushed (for being WWE or NWA rejects).
I'm not saying WCW circa 1994 was identical to TNA circa 2013, but the similarities are there. The only difference is that now WWE has contracts in place to make sure there's at least 90 days before anybody can jump ship. So guys can go from WWE to TNA, but they can't just leave Raw on Monday and show up to Impact on Thursday to pop the ratings. But CM Punk is the kind of guy that could go away for 90 days - or even a full year - and show up on TNA and bring in the biggest ratings of all time. Maybe he's not Hulk Hogan, but he's sure as hell Razor Ramon. He could jump that ship and take TNA to new places of relevancy. ESPN would be all over it. THe dirt sheets would explode. Impact would hit a 1.2 in the ratings (lol).
Just saying...don't ever say "never" in pro wrestling.