Your points are well taken. I guess the reality is that 20+ may be the critical mass. Raise the price, and for every person you add, you might subtract one. Keep the price low, and you chase people away who are "afraid" of an 86 pt wine (although for the life of me, I'll never understand this. IMO, it's worth the price of admission just to participate in the experiment).
My only quarrel with your position is that if the host selects a huge production wine, quality will suffer. As Brian points out, most TAAs have scored about 86-87 pts. A host could pick an entry level Beringer, a Meridian, a base level Ch St. Jean or a Simi and get the same scores. Virtually everyone would be able to find and afford these. But I admit, interest would be limited.
Brian: The TAA can never promise to be a 90+ wine that sells for $18. The whole point (I think) is to try out a wine that has a better than average track record, is affordable, and is available. If it proves to be better than expected, then it's a winner. The TAA also serves as an educational experience to allow a broad base of forumites to opine on and rate the same wine. I also like the metaphysical notion that as my wife and I sip the wine, we know that 50 others are doing the same thing at the same time. But if your expectations are solely that the host has scoured the earth to find a hidden 90 point gem that no one has rated, and no one else knows about, it just ain't gonna happen.