They'll just go on to make the Fallout MMO forever. MMO fans are unquenchable in their thirst for more, so that'll sustain Bethesda forever, and they can focus the rest of their efforts on the Elder Scrolls, which will remain as it is because as evidenced by Skyrim, it sells. That, or they move onto MMOs for both IPs. Realise that Konami threw away their entire gaming history in favour of pachinkos and are still making money. There's always someone to sell to, as long as you've got the right thing to sell.
If we're looking for a possibility that Bethesda might collapse and give away the game, ha ha, good luck with that. Konami is still clinging on to Metal Gear Solid. Just because a company will never use an IP doesn't mean they won't just hoard it all to themselves.