I'd lean more on the side of calling it a Ponzi, but I think in either case it'll only be clear in retrospect.

It'll either:

1) Stagnate. You'll need to wait a really long time for answer.

2) Crash or trend to zero. This would mean momentum is exhausted and money is leaving the table - it's always been a Ponzi in this case.

3) Continue until it finds a utility or it genuinely becomes a store of value. I doubt this outcome very much (I actually think it's impossible due to inherent features of bitcoin), but what do I know? I'm just one person on the internet. Not a Ponzi in this case.

It's not entirely a ponzi scheme. But what remains is crime.