Then the network can be attacked by hard forks.  New nodes (or nodes temporarily offline) will see a reorg of greater than 720 blocks as valid and be permanently forked from the nodes who were online and saw the reorg as invalid because it was too deep.  Non deterministic behavior of nodes is something to be avoided, to achieve consensus all nodes must reach the same conclusions on which chain is the "best".


The attacker has no limit on how long it takes to acquire the coins.  The clock starts from the point of the reorg which can be just prior to selling the coins.  So if attacker has x+1 coins where the network stake is 2x at block y he can start the attack chain from there.  He now has 720 blocks in which to record the sale/transfer of the coins for material gain and produce a longer chain.

The attacker isn't limited to buying "valid" coins, just the "history" of coins that a private key had in the past.  A private key which at one point had unspent outputs worth x coins but today has no unspent outputs ("zero  balance address") has no direct value to the owner but to an attacker it has value in attacking the network.  "Hey large coin holders I will buy your empty wallets based for 0.1% of the coins they had as of block y".

The attack isn't limited to 720 blocks.  A reorg of longer than 720 block is possible it just once be accepted by the entire network.  Permanently forking the network is still a powerful attack, especially for one which has no cost or risk.

The fact that you are moving the goal posts from "the history can't be used" to "this would be hard" (so is a 51% PoW attack) is good enough for me.  I think you do now see that someone can attack the network without cost or risk which was the point you refuted as false.  How difficult it would be to acquire that stake which is a totally different argument.