Coinbase makes more money by being a trustworthy exchange than by skimming the funds from the USDC reserve.
Some bright spark will notice that they don't need all these untouched reserves and will do something with them, increasing the risk and, eventually, getting hit with a bank run.
There should be by then other mechanisms to get liquidity and that can represent the economic output of the world. It could be a basket of CBDCs from multiple countries, or tokenized (and legally standing) representations of bonds, titles and property. But the current system is far from the final design.
Basically every sizable payment processor for instance meets that bar and it’s incredibly rare to have a scandal with those institutions.
Fractional reserve banking isn't a scandal, it is routine business.