I've always considered technical debt as analogous to the ongoing work required to maintain a house. That is, it's usually not possible to bring everything up to best standards at once. Priorities have to be established. We've already seen what large technical areas will get focused attention for the next year or two - and they're ones that are much more heavily used, much more critical to the function of the overall Wikimedia projects, than Special:Book, an extension that was mainly used only by (some) Wikimedians when it was fully functional. This is the correct priority. A volunteer could, if they were so motivated, spend some time on Special:Book, perhaps even get a grant to do so. But addressing the serious, mission-critical issues of Commons beats Special:Book every day of the week and twice on Sunday.
Generally speaking, issues involving security, system integrity and the functions used by the widest segment of our audience (which includes Wikimedians, of course), would logically be of higher priority than a useful but (comparatively) little used function. In another thread, there was mention of there being 50,000 "books" attached to English Wikipedia. We have individual images on Commons that probably have been used more often than that (although many of the uses may be by third parties).
Risker/Anne