Even at its downs, bitcoin has found demand still. That is a very interesting signal. If it were just the ponzi as you say, it would have come crashing down on its downturns.That is profoundly faulty reasoning.
For a lot of people, a drop in bitcoin is just a signal to buy because they have an irrational belief that it can never really go down permanently (which, to anyone who remembers 2007, should sound familiar).
A ponzi scheme can go on as long as you can find new market entrants, and what makes Bitcoin remarkable is that it can tap into revenue sources throughout the entire world, and in particular in places where socioeconomics and geopolitics makes something like Bitcoin extremely attractive. That's a lot of fuel for this particular fire to burn before it fizzles out (presuming fundamentals aren't established before then).
The world has never really seen anything quite like this, and it'll be fascinating to see what happens over the next 10-15 years. And, meanwhile, again, there is a lot of wealth being transferred, and I'd love to see where it's going...