A quick google search(so take this number with a grain of salt) tells me that reddit has 52 million daily active users and 430 million monthly active users monthly active users.
Reddit is BIG. Really big. And websites just dont die or pop up the way they used to because of how theyre designed and because of how the current era of the internet has matured.
That said I wouldnt call my take pessimistic. Reddit has gotten too big and there is an appetite for many users to leave for greener pastures but there just wasnt enough momentum to breath more life and activity into the alternatives. New reddit will chug on but enough people will leave that alternatives will sprout and grow. Whether that is Lemmy or something else it's hard to say this early on, but it means that reddit alternatives will have enough users to thrive.