There are over 40k tumbleweed badge-giving questions now on SO (41,959 on last check), out of 186,432 unanswered questions. SEDE (although a bit dated) lists 974,000 questions asked, so that's almost 20% of questions unanswered, and 4% doing worse than unanswered: they got ignored. The nature of the tumbleweed badge being that it is awarded once, there are potentially more than 4% of questions that go ignored.
Maybe we're growing beyond what the system as designed can handle. Lots of these tumbleweeds are from 1 rep users, and I'm betting that's turning off a lot of potential answers. Still, there's a growing (is it also growing proportion-wise?) pool of unanswered questions that are getting no attention.
To up the value of answering tumbleweed (or tumbleweed-worthy) questions. When a question has the criteria for the tumbleweed badge (low views, 0 votes, 0 answers):
- Upvote on answer give more rep (15 instead of 10), until there's an accepted answer
- Accepted answer gets more rep (30 instead of 15)
Some suggestions have already been made in the past (for example auto-rep bounty) which would arguably be gamed for maximum profit, which would have all sorts of perverse effects. Likewise if we had an answer reward proportional to question upvotes. These are out of the question. But to force a question to tumbleweed so it can be worth more to answer it? I don't know how that would get done. Just looking at the question to see if it's content is tumbleweed-worthy is bringing it closer to not getting tumbleweed.
I believe we can up the incentive on the forgotten questions and produce a lot more satisfaction for users everywhere. (And none of my questions ever tumbleweeded, so I'm not doing this for me :P)