It would follow the same route as physical albums/cds.
If the label goes tits up then there will be someone waiting to buy the rights of some/all of their catalogue. The new label will release the stuff.
The same would apply for online albums - label goes bust, new owners come in and buy what they want and release them the way they want. Hell, they might even release an online only album on CD too :-)
There's always another company wanting rights to stuff that can make them money wether it's physical or online.
