I've heard this one many times. Bu everyone now still has to agree to enter the transaction on some specific block chain. Even if you limit things to just the us. The issue isn't that we don't have the tech to have a national database. It's that the records are scattered all over dusty county and town clerks offices in thousands of jurisdictions for all sorts of historical and political reasons.
Most municipalities have digital records available. It ought to take less time to translate those to blockchain records than the time it took for paper documents to be digitized.
As for the selection of blockchain, that's part of where the current speculative wave in the technology comes in. Competition will persist until there are only a handful of well-defined players, at which point it will be reasonable for governments to tie records to them. All it takes is time.
What about a deed or title for property? Which of countless organizations and public records sources has the document? If it was entered on the Ethereum blockchain, there's no doubt as to ownership.
There's also tracking of goods; outbreak of salmonella? If the trail is available on a blockchain then it's easy to find the source and anything else that might be tainted.
The only reason crypto is volatile right now is due to speculation on which blockchain(s) will be most successful, just as TCP/IP was not always the foremost protocol. It is the nature of growth to be uncertain and wild, and be certain that there is no going backward: Bitcoin and blockchains are here to stay.
One additional note is that they are incredibly powerful tools for control and tracking - a deep state's wet dream, in the process of being willfully adopted by the masses and soon to be the delineating mark between the haves and have-nots. Make no mistake: blockchains can be manipulated by institutional forces. Huxley and Orwell could not have imagined this in their worst nightmares... beware.