And CLEARLY the post-series novels are selling. Pocket is a business at the end of the day, and if they weren't turning a profit on the novels they were selling, they wouldn't be ordering more novels that take place post-series.
Exactly. Numbered DS9 and VOY novels set during the canonical series' runs were not doing too well, IIRC. There were a few spikes (eg. Christie Golden's VOY titles, which is how she got the VOY Relaunch). I think the short story anniversary collections for each series did quite well. "String Theory" seemed successful, too, but that trilogy also had its critics.
And, since the days of John Ordover, Pocket has seemingly found marketing success when related novels are released in clumps. While it sounds fair to have twelve different series all getting one month each per year, having a concentration of like titles seemed to be a more viable model.
As for TOS, for a long time it seemed like we got too many TV series era books and not enough set in the movie era, but that's evened out a bit over time, despite the fact that several critically-acclaimed movie era novels did not achieve the numbers expected of them.