I haven't seen people talking about the new Battlestar Galactica series after it ended, either. Often, once "the answer" exists, people stop wondering what it is.

This point is surely correct, but you again pick an unfortunate example - I've heard the ending of BSG was even worse then DT's...

you almost never get that kind of coordination when it comes to "old" works.

Which is interesting, since there's nothing stopping a group from just not reading each & every book after a set period, thereby reaping the same gains but without issues like, I dunno, the author dying after 20 years & leaving it incomplete. (cough Wheel of Time cough)

The fact that people never do this, even in private, but rather prefer to tear through the entire series at once, suggests to me that this communality isn't worth much. (Aren't book clubs famous for falling apart after a little while?)

Perhaps the fans are just distracting themselves from the agony of waiting for something they love so much & killing time; I knew, before & during the prequels, more than one Star Wars fan who just tried to ignore anything they saw related to SW so they couldn't be bothered by the multi-year waits (out of sight, out of mind...) - they felt the itch you get when pausing a movie or show, or stopping in the middle of a book, but this itch would last for more than just a few minutes.