my thoughts on this are that it's a good idea, but it can't be the only solution. moving from a single point of failure to a more secure single point of failure is okay, but a better solution would be for the Library of Congress and similar public bodies around the world to be able to buy/obtain a copy of the archive and the protocols it uses to do the archiving. this would practically guarantee the permanency of the data and, if it was deemed legally viable, provide a decent revenue source
It's alway funny to see people suggest things the federal government should do without considering its deliberately limited nature and why that should go out the window with a constitutional amendment. This isn't the UK where we can just make up the law however we want. 10th amendment anyone? Enumerated powers?
Only to the extent that the library of congress is necessary and proper to regulating interstate commerce are its activities authorized. As soon as it strays from that, it must find another enumerated power with which to justify its actions. For instance, if the library of congress were to begin deporting illegal aliens, that would be incontrovertibly within the enumerated powers. To say nothing of the will of Congress.
A centralized copyright register is obviously something important to interstate commerce. Existing internet archives do not keep track of whose copyright is whose. So to transfer ownership of such a thing to the library of congress would not serve any copyright function related to interstate commerce. Surely even you can see that. So you're going to have to get real fucking creative, buddy!