I'd like to point out, though, that we do expect there to be an endpoint to a western novel series, even if it's far away and takes long time to reach (Wheel of time...) and people who just keep churning shit out (Terry Goodkind) are regarded as hacks, so it's not like this view is unique to manga readers.
Thaaaaat depends a
lot on the genre, from what I've seen. Endpoints are not particularly expected in a lot of romance/western/etc. novel series. Being unfair, basically nothing except barely (or not at all) veiled smut is expected from a lot of those, but still :V
There's also plenty of stuff aimed at young adults* that just kinda' spools out until people stop buying it.
* A generation back, you'd be looking at goosebumps, hardy boys, etc. Zog if I know what's rolling around nowadays, it's been the better part of a decade since I've been inside a non-used bookstore, and I didn't even pay much attention to those things even when I
was in the targeted age range. It's definitely pretty easy to see at a glance in libraries and such that there's a lot of areas on the shelves with thin-ish books lined up next to each other with similar spines, though.