The scenario was already reality at least once. MtGox got hacked in 2012 already (before it was sold to Mark Karpeles actually), but they've hidden that fact until they went bankrupt in 2015 because they thought there would be a way to re-buy them with trading bots, this failed miserably. If I remember corretly there were thus around 400,000 "additional" GoxBTC shown in the exchange's user balances and order books. The Bitcoin trading volume wasn't as big at that time as today, so these 400,000 additional BTC probably had some influence on the market.
There may be a scenario where large tradfi institutions are making tons of fees from volume trades of BTC derivatives that never hit the blockchain, and they simply use BTC as a settlement layer occasionally for 10Q/10K filings.
I think the incentives do not really align in that direction. Layer-2s are very cheap fee-wise, and once they become really easy to use (I expect that to be the case when technologies like Ark and sidechains reach maturity, because LN unfortunately has some "conceptual" weaknesses) then they will compete directly with Binance Pay, Coinbase and friends. The money which could be made from fees would then probably not be very important, with the exception of trading fees, because trading is a technology where centralized entities will always have an edge compared to DEXes due to their speed / efficiency.
Layer-2s are easy to audit if they're blockchain based (almost as easy as Bitcoin), so the banks and TradFi institutions would need to offer something to attract users. So they would need to build up trust with credible audits.
This would of course not mean that something like MtGox could not happen again, because there are still loopholes, but it would be probably temporary and not by a large amount.
A technique which still wasn't mentioned is Bitcoin-based stablecoins without Bitcoin backing. Think of Dai but pegged to Bitcoin. This was already tried out by BitShares in 2015 or so, called BitBTC, but failed to get traction and is now dead, I don't know if there have been other attempts. This is indeed a way to increase the supply of Bitcoin-based "money tokens" in a completely legitimate way. But it's auditable like any "layer 2" would be, and the supply is also limited by incentives, if it gets too high then the token will probably collapse without taking Bitcoin down with it. These tokens are not BItcoin, they are speculative CFD contracts.
Thus I would say that the very popular NYKNYB phrase can be changed even to "Not your [Bitcoin] keys,
not Bitcoin".