Isn't it contraproductive to link Alice's old Bitcoin to the Bitcoin she receives in the blockchain?
If the coins are locked until another transaction has transpired, miners (and therefore everybody) will see that, even after said transaction has transpired.

People can search the blockchain to see if there was ever a condition that stated x Bitcoin had to be sent to Alice's address in order to unlock Bitcoins. Leaving Alices new 'mixed' bitcoin forever linked to her old ones.

Or do I see this wrong?