The FDIC is not an absolute guarantee. The government could decide that it doesn't want to honour it, or could itself collapse. Both scenarios are very unlikely to happen in the near term, but the same is true for the largest and most stable banks.
It’s probably the most fundamental axiomatic assumption underlying any normal discussion of financial risk.
In other countries, it isn’t. “How can we manage a complete collapse of the Filipino government?” Is a reasonable question.
If the US government collapses, Coinbase won’t have a market to operate in so further discussion doesn’t matter.
If you round the risk of the US government collapsing (or refusing to honour its obligations) down to zero, you should probably do the same for many private banks.