I disagree with the premise that the TP site has failed. We have succeeded in maintaining high quality site which has become a reliable resource of correct and useful information, which is sometimes not that easy to obtain. We had top people coming to the site and contributing high quality content. Quality, rather than quantity, is of course something that is not easily measurable, but I think it has value nonetheless. Traffic was an issue, but for a highly technical site the pattern of traffic, including how it changes with time, should be evaluated differently. Certainly things could have been done better (e.g. Encouraging more explicitly student questions, as long as they don't have standard textbook answers), maybe if this experiment continues elsewhere sometime...
I have to say that it is a bit disheartening to discover here, as rationale for closing the site, a lot of the preconceived notions that people had all along (e.g. That it has large everlap with the physics site, which is demonstrably false). I guess it is not that easy to change people's opinions.
My personal conclusion is that an isolated, highly technical community like ours needs to maintain their own infrastructure. SE network is great in general, but the large body of rules and opinions on about how things should be done was sometimes less than completely helpful. Maybe they are simply not set up for maintaining highly specialized sites. Perhaps an independent site, like math overflow is the way to go.