What do you mean by "social consensus chain"?
Any UTXO that you create now, when the mining difficulty is quite high, is going to have a very high security, even if the mining difficulty goes down afterwards.
Any UTXO that you create now, when the mining difficulty is quite high, is going to have a very high security, even if the mining difficulty goes down afterwards.
As the difficulty gets lower, the number of transactions to drop from the top gets higher, and maybe also less certain.
It makes no difference about the mining difficulty in the past. If double-spends are possible _today_ then you can't ever prove you own the coin, regardless of whether you could do it in the past.