I'm not following your logic here. If there's 10 people each with a $2.5m deposit at a separate bank each and they all spread their accounts to $250k in each of the 10 banks, the FDIC still has a risk of $2.5m per bank right?
The cost to FDIC if an individual bank fails is the same in both the above scenarios, even though in the first businesses put a lot more effort into spreading out their funds. It looks less like it could have less risk to the FDIC, but really isn’t making any difference.