It's orders of magnitude more likely for a series to be discontinued by the publisher due to poor sales than because the author is taking too long to produce the next volume. Martin and Rothfuss became so infamous for this exactly because it's so rare. On the other hand, midlist authors get their series cancelled before their completion all the time.

Anyway, personally I find this worry strange. For me if a volume in a series is not worth reading unless it gets a sequel, it's not worth reading at all.