It's really simple. Exchanges just need to respect users' funds. If you have 1 BTC deposited on the platform, they must keep that 1 BTC there, instead of exchanging it into altcoins to run their schemes.
I am not sure but ignoring this aspect was the primary cause behind what happened to FTX.
With the proof of reserve, that's what making everyone to think that their funds are safe into exchanges, mainly in Binance. But, with that idea also, everyone must be aware right now that if you deposit an exact btc or any altcoin on them, they're using it somewhere else for their personal gain.
Proof of reserve should be that if there are 100,000
BTC deposited by clients, there should also be 100,000
BTC kept in a safe storage. There's big trouble when clients deposited a total of 100,000
BTC and the exchange simply says it has $1,683,140,000 worth of assets that back it.
There's even bigger trouble when 100,000
BTC is supported, still by an equivalent worth of assets, only that it is in their own tokens. Alert bells should be ringing extremely loud. So, if 100,000
BTC is supported by $1,683,140,000 in FTT or BNB or BUSD or KCS or HT or whatever exchange token, everybody should start worrying.