The alternative is to let people quit for months until prepatch, which is a very risky move. Blizzard knows that their biggest hook is that players that are currently playing the game tend to stick around, and ones that put it down (if even for 30-60 days), are significantly more likely to never return to hop back on the hamster wheel. It's a game that relies heavily on sunk cost.
There's a good chance that not giving them SOMETHING new to do between now and the 30 days prior to launch (let's say it's December), gets them playing their new shiny Xbox/PS5 if they were lucky enough to snag a pre order. Gets them playing Valhalla, CoD, and Cyberpunk, and makes them significantly less likely to continue playing their expansion.