It boils down to you can't expect a return on bitcoin hence it's shouldn't be worth anything. If that argument was true, then gold is a ponzi that shouldn't be worth anything and the same would apply to art.
A slightly more advanced rebuttal would be to say, yes but gold has some industrial use and you pay for art because you can enjoy consuming it. But the price of gold and rare artistic pieces is many times their "useful value" if defined as such.
All these arguments show a deep misunderstanding of what constitute money, the history of money [1] and the concept of "hard money". Bitcoin is the only way to transfer an arbitrary amount of money in a way that is fast, verifiable, permissionless and to an arbitrary location. This particular use, that no other tool makes possible, means it has some value to some people and consequently should be worth something. The question of what its fair value is harder to answer but just because gold is worth more than its industrial value doesn't make it a ponzi scheme.
[1] https://econwpa.ub.uni-muenchen.de/econ-wp/eh/papers/0211/02...