It would have been far cheaper just to rescue the FDIC and preserve the money held in savings/checking accounts.
Other countries which offered guarantees on savings accounts still bailed out banks due to the wider economic issues.
Credit would go from "hard" to "near-impossible". Investment banks finance all sorts of things you would never think about. For example the kitchen equipment in your local fast food chain (The pizza chain Dominos ended up buying the leasing arm of an investment bank which financed it's kitchens to ensure it's own stability).
Larger companies rely on their investment banks for all sorts of things, from FX to protecting against counter-party risk (i.e. providing insurance against your main customers or suppliers going bankrupt).