The problem is not that it was "Just one of the things that died out because no-one could be bothered to maintain it", it is worse: it was broken on purpose, and not recovered, because the WMF decided that no one cares about it.
That is patently untrue. The book renderer (OCG) was, due to the lack of maintenance, increasingly causing problems for the operators of Wikimedia production services, and the approach it was based on (converting wikitext to LaTeX) resulted in an endless stream of discrepancies in the PDF output. It was replaced with another PDF rendering service that used a headless browser - an approach that resulted in much more faithful rendering (basically it outsourced the cost of maintaining a good PDF generator to browser vendors) but didn't scale well and wouldn't have been able to handle large collections of articles.
I'm not fond of that decision but it obviously wasn't about disabling something that worked before, just for fun. The Foundation had to choose between risking platform stability, a significant time investment to modernize the service (at the detriment of other projects that time could be invested into), and shutting down a feature that saw relatively little use, and chose the third.