Since we're discussing how the Tech Wishlist works then I will comment on a few points specifically regarding that wishlist.
1. A gentle correction: the recommendations are ranked by vote, not by consensus. This has pros and cons.
2a. If memory serves me correctly, the wishlist process was designed by WMF rather than designed by community consensus. I may be wrong about this, but in my search of historical records I have not found evidence to the contrary. I think that redesigning the process would be worth considering, and I hope that a redesign would help to account for the types of needs that bawolff described in his second paragraph.
2b.. I think that it's an overstatement to say that "nobody ever votes for maintenance until its way too late and everything is about to explode". I think that many non-WMF people are aware of our backlogs, the endless requests for help and conflict resolution, and the many challenges of maintaining what we have with the current population of skilled and good faith non-WMF people. However, I have the impression that there is a common *tendency* among humans in general to chase shiny new features instead of doing mostly thankless work, and I agree that the tech wishlist is unlikely even in a redesigned form to be well suited for long term planning. I think that WMF's strategy process may be a better way to plan for the long term, including for maintenance activities that are mostly thankless and do not necessarily correlate with increasing someone's personal power, making their resume look better, or having fun. Fortunately the volunteer mentality of many non-WMF people means that we do have people who are willing to do mostly thankless, mundane, and/or stressful work, and I think that some of us feel that our work is important for maintaining the encyclopedia even when we do not enjoy it, but we have a finite supply of time from such people.