Thanks for the explanation PVMiner. Still I do not understand why it would add any complexity if the transaction got dropped lets say 1 week after being initially broadcasted.
Because there are no good ways to enforce it. And if you cannot enforce it, it's better not to have a false idea that there is such a rule. The current rule in the Core is that transactions older than 14 days are removed from the mempool. But you cannot be sure if anyone follows that rule.
Enforcing the rule would make it worse not better. Because you would have to remember that a certain transaction happened at certain time. And by doing that you are opening a possibility of a denial or service attack by flooding the network with many transactions. Currently if you flood the network, the nodes will remove the lowest-fee-per-byte transactions and you cannot exhaust the nodes memory.