Although far too many critics believe that Sword Art Online should have remained in Aincrad, the franchise's final war in Sword Art Online: Alicization War of the Underworld, set in SAO's fourth MMO environment, Underworld, could never have been as grand or as far-reaching as it was if author Reki Kawahara complied with these critiques. In fact, Kirito's final battle against Akihiko Kayaba in Aincrad can be easily interpreted as what Sword Art Online's finale could have been if the story didn't suddenly move into ALfheim Online (ALO) midway through season one, which would have been underwhelming by comparison to the true ending that Kawahara envisioned.
https://screenrant.com/sword-art-online-alicization-final-war-underworld-improved-series/