It was revealed in season 4 that Vecna was the mastermind behind the Upside-Down's incursion throughout the show's events, so he may have intentionally kept Will alive. It's still a mystery as to how he was able to survive the ordeal without proper sustenance. However, now that fans have found out that Vecna is the self-proclaimed leader of the Upside Down, it means he not only knew about Will's abduction but must have organized it himself. The ending of season 4 also revealed that Will has a connection with Vecna. It's possible that Vecna wants to keep a connection to the human world through Will and could even want a human host.