You buy a house, the transaction gets on the blockchain network. Just like you wait for confirmations with Bitcoin you'd wait e.g. for 1 day or for 1 week confirmations, this would be agreed upon by both the seller and buyer.
Now, let's say someone with deep pockets re-computes the last 1 week of transactions in a 51% attack and "undoes" that transaction. The branch of the blockchain that got replaced doesn't just disappear, it has your house in it, thousands of other houses will have been sold through that branch of the chain, and thousands of people will have that full public record.
At this point you and the rest of the owners could obviously go to a court and get your houses back, just as you could if the deeds got lost through a fire today but you had other evidence to prove they existed.
Nobody's suggesting that the entire court system be replaced by a blockchain and we ignore all other mitigating evidence, it's just being suggested that storing e.g. deeds in such a system is more reliable than storing them in a couple of filing cabinets somewhere.
Meatspace resolution is a feature, not a bug. Including for cryptocurrency, as the ETH/ETC hardfork has definitively shown (comparing the relative values of the two). Everyone talks a big talk about immutability... but nobody actually wants to live on the "authoritative" blockchain where somebody made off with 15% of the money supply.
If your assumption here is that you live in a war-torn country where a warlord can use guns to take away your property, or another situation with no real rule of law... the warlord is not going to care about what some "blockchain" says. Or they'll use their guns to force you to transfer it to them for a penny. Get real.