They're putting the cost, presently unknown and probably not huge, on the other banks. But the message to depositors everywhere, of every size, is "don't worry about your bank's solvency, we'll protect you".
So market scrutiny is removed as a discipline on bank asset strategy. That leaves regulation as the only control. That politicizes and bureaucratizes bank lending. And the general presumption that Big Government will protect you from yourself is extended just another smidge.
If you had the wit to think "hey, maybe I should be careful with $5 million" and bothered to put it in T-bills or an insured sweep, you're just a nerd who should know the Government will take care of such things.
It isn't the end of the world, but it's a sign of how corrupt and complacent we've become.