Some people forget isekai is pretty old. For late 80s and 90s kids, Digimon and Inuyasha are isekai (well Inuyasha is closer to portal fantasy but isekai on its own is a sub branch of that).

Hell, Spirited away is isekai, and i can bet most people watched that before SAO. SAO just happened to be the most popular isekai at the time that isekai became mainstream and abundant.