Why should a prediction of demise (for whatever meaning of "demise" we care to choose, but we can start with the one you first mentioned, market capitalization) be required to be more precisely detailed about how and why than a prediction of thriving?

Someone could say "Many past novel currencies and securities have failed; any prediction of Bitcoin's success must explain precisely how and why it will still have worth in 50 years." To me this seems like the same argument you're making - belief that a trend will continue until proven otherwise - but I don't find either of them very convincing.

I think any argument either way should have more details than "it's worked so far!" or "it'll eventually fall over like Madoff!" but you seem to only be pushing the burden of proof in one direction.