[Taek]

The security isn't there to protect historic transactions, it's there to protect the holdings of current holders. If I had a bitcoin at one point and no longer have that bitcoin, I'm completely indifferent about the security of the transactions involving that bitcoin because it no longer impacts me.

Also, the cost per transaction - even if viewed through that lens - does not approach infinity as time goes to infinity because there would be an infinite number of transactions as well.