That sounds like a very specific hypothetical future risk. That's a logical outcome in a vacuum, but companies have managed large cash and near-cash positions for a long time without services like you describe. I think your "slippery slope" argument is a fantasy.

But if it weren't, and if it were a problem for the FDIC, which I don't think is established, then the FDIC can just change the definition of what's covered a bit. It's not a hard fix for them to say that they won't cover more than $x per beneficiary total.