I honestly dont understand why it cant be classed as a gamble. Some players win, the majority lose.
The crossover comes between an actually Ponzi scheme, which by definition will eventually die, and an owner who is just looking to make a quick buck by running off with a high wallet balance.
Gambling involves a stated definition of odds. When I play blackjack, I know the odds. When I play craps, I know the odds. When I play VP or slots, I'm legally allowed to know the hold percentage. Dice game have stated odds. Shit, when you buy scratchers, you can look on the back and see the odds.
Schemes don't have odds. Because they are not fair. If they said "82.5% of the time, you will 1.2x your money", and had a provably fair way to decide who won and lost, that would be gambling. They don't. You just have to hope that the owner decides to not run off with your money AND that enough suckers will invest after you to get a return. That's not gambling.