(Note that there is no way for a miner to determine when a transaction T1 was signed. Even if it spends an UTXO in a transaction T2 that was confirmed only yesterday, it is possible that both T1 and T2 were signed a long time ago.)
Your argument is technically specious. Transactions in Bitcoin have 4 byte version field, that gives us potential for billions of rule-sets to apply to the old transactions. The correct question to ask: why this wasn't and isn't changed as the rules gets changed?
I am not sure if I understood your comment. Miners cannot apply old semantics when the transaction has an old version field, because that field can be faked by the clients to sabotage the change. E.g., suppose that the change imposed a mininum output amount of 0.0001 BTC as a way to reduce spam attacks on the UTXO database. An attacker could frustrate that measure by issuing transactions with the pre-fork version tag. Does that answer your comment?