Please don't compare Carey to the Theodore syndrom.
Jack Gelineau and Frank McCool are your best comparables since they both disappear after winning their Vezina in the '40's.
As for Carey, he simply had problems moving but he could have been a good backup.
After winning the Vezina, his problems began during the playoffs. Also he faced though competition within his organization with the emergence of Kolzig and Dafoe. So he got traded in the blockbuster deal we all know.
He arrived in Boston, and the Bruins were brutal, he was completely awful at the end of '96-'97.
In '97-98 he was ok at the beginnign of the season going 3-2-1 with a ,893 save percentage but his job got stolen by Byron Dafoe a newcomer in the organization. Plus Robbie Tallas did well the season before and therefore the unhappy Carey who lost his #1 job got sent down in Providence were he faced some injuries. Carey was earning a lot of money with his contract, I think that Boston saved some money sending him in the minors (not sure of this)
In '98-99 he did very well in the AHL but was trapped because of his contract, he could not go up because of his salary.
St-Louis gave him a chance in '98-99 but Carey completely blew it and he walked away afterwards. I believe he felt his days in the NHL were over.
I think Carey could have stayed in the NHL if he had a better contract but for sure I do not believe he could have stayed a number one goalie.