In hindsight, I feel what Kawahara really wanted to write was a reincarnation/isekai story, but 2009 was before isekai was invented? Or in vogue? There are "stuck in an MMO" stories where the MMO is the important part (.hack, Log Horizon), and there are those stories where the MMO isn't important at all (Ready Player One, Sword Art Online). The latter kinds of stories eventually cut out the MMO part entirely and just throw their dimwitted protagonists into a vidya gaem world now. (Konosuba, Danmachi)
This, in my mind, explains why Kayaba is little more than a walking plot device/self insert. Kawahara couldn't care less why they were in SAO, just that they were there, making it functionally identical to isekai.