Interesting points, i think older bats probably feel good as they are soft but this doesnt mean they are the best performing.
Newer bats will feel harder but will likely have more punch to them.
I'd say after the first couple hundred runs or so is when a bats in its prime from my experience.
So how does knocking in work? It hardens the face of the bat by compressing the willow to a depth of a couple of millimetres. Using the bat then adds to this compression. While the bat is in this state it is not soft.
Use it some more and you will start to destroy the cell structure and crack the "crust". This is your soft point. I can assure you the use of a bat in this state does not feel good. En equally, the new bat is not as hard as the knocked on bat as this annealing process has not yet taken place.
Ergo, a bat used by a tail ender over a number of years is likely to be nicely knocked in as it won't have scored huge runs and as long as it was treated properly will be perfect for use, and cheap!