This isn't remotely shocking... indeed about 5 years ago, he did an interview where he expressly said "I'll be out of wrestling in about 5 years". That, more than anything was perhaps why he wasn't backed all the way by WWE, rather than any injury proneness.
After all his two "major" injuries came at the hands of others, who then took the push he was intended to get... so WWE would have a VERY hard time holding that against him unless they'd punished those who caused the problem... so they persevered with their half hearted attempts at pushing him, partially to convince him to stay as he does have value to them as a character, if not a top level worker and partially like that awkward ex you just have to be nice to cos your buddy crashed the car and scarred them to the point you couldn't be with them... it wasn't really their fault it went wrong.
The first signal was a few months back when he reverted to his real name on Twitter, that was when the LoN came about and that seemed like a "last pitch" from WWE.
Barrett or rather Stu Sanders has quite a few options. Bear in mind massive openings have just begun in Japan... with AJ, Balor and the Bullet Club now over in WWE their top new guy is Kenny Omega? Kenny, Stu has some bad news... he could go over there and easily earn close to what AJ did as a 5 time IC champion and potentially even get the IWGP title... he'd be a hot heel gaijin and not even need to wrestle that often to do so...
Couple that with TNA, where he would walk into the World title picture or even GFW where he could be the first "big steal" that helps put them on the map along with cameos in ICW and you still have a very lucrative Indy career.
However the elephant in the room is the "nerve damage" in his neck... remember this is a guy who lost a good part of his strength, which was an original selling point due to Ziggler's negligent bump snapping his arm at the elbow... he always struggled after that, moves like Wasteland always looked riskier cos he didn't have that same stability in his lifting. Add nerve damage to that equation and it might simply be a case of not worth the risk anymore.
But even if that's the case, Sanders still has a lot of options... He has been in some movies, albiet WWE ones, but he has worked with some big names in doing so... He could get an agent relatively easily and with connections like Batista, The Rock all knowing how good he is could easily appear in an Expendables type movie down the line as "British Villain". Meanwhile, his sense of humor would lend him to coming over to the UK for shows like "A League Of Their Own", which is a sport based celeb panel game presented by James Cordon... He'd fit perfectly with their "crazy" humor on that show, the wacky games and banter... he'd excel in that environment. The guy could easily end up presenting here in the UK and potentially in the US, he's like Jericho but with the added Brit accent which will make him more attractive to networks.
Whatever happens, it won't be the last time we see this guy... There is even a chance that after a period of time WWE decide they want to make him an offer to be that commentator guy we all think he should be... once Jerry hangs up his mic for example.. but in the short term, there just isn't that spot available.
But ultiimately WWE won't want to lose him but they can if they must, he was the first product of NXT in any form that "made a spalsh" and arguably set the tone for what it became... he is a 5 time IC champ which is the same as Razor was.. he has a 6 year WWE career that when he was good, was VERY good and the lows can very rarely be blamed on him... He's the kind of guy WWE gives another go to even if they walk...