That's not how a ponzi works, it's completely different.
First, they don't have to pay someone from the funds the first investors put in, why would they?
They are getting money for a product, why would you risk your business to not buy it when you could simply buy the coins and still make a profit from the commissions you take from your clients? What would be the point?
BR made billions this way, they don't care about the value of those assets, that's what investors should do, up and or down BR just like CEX makes money!
Even if Bitocin would drop to 10k or raise to 100k, BR would still make money!
As you say, don't trust, verify, Is there any address where we can see the actual BTC stored? How can we know it is the BTC for ETFs and not just the BTC of the clients of CoinBase?
So, did you question also Microstrategy's finances? Did you question also Bukele funds?
You know for both the custodian is also Bitgo and Coinbase, right?

But wait, those were the "good" guys, right?

Either way, we end up trusting somebody, as we don't have 100% proof.
And since you have no proof just accusations, ....see where this is heading?
