I'm not sure what your proposed alternative is? If you don't peg the payouts to crypto FMV at the time of the crash, then FTX is still insolvent (because their debts were also denominated in crypto), so nobody gets anything.
The argument is that equity should get nothing, and the crypto-holder's assets should be adjusted for current value... just like how cash-holder's recovery would be adjusted for inflation.
Is it really?
Till v. SCS Credit Corp, 541 US 465, 474 (2004).
Whether the claim is on cash or collateral doesn't seem to have figured into the law.
[1] https://bbklaw.com/resources/best-in-law-which-interest-rate...