I know this is from the other day, but if they did that, expansions would have to be shrunk down in scale considerably. We're talking like, an expansion that releases with half as many zones, dungeons, and raids at a minimum in order to do that, and they seem incapable of doing so.
I also feel like it would just... not be very fun, given how they tend to tell their stories these days. Personally I feel like two years, or at least a year and a half, sixteen months, somewhere around there? Something like that would be more reasonable. It all depends on how long it takes to create certain the various bits and pieces of the game.
I don't think we'll ever see the 77 day release cycle of Legion again though, that's for certain.