You have proven nothing. There is no market for formerly large stakeholding keys. But you claim by declaration that they are easy to get and at significantly below market price. And that the current mainchain will just magically switch to the attackers chain.
Making a wrong statement is not quite the usual standard that comes with the word "proof"
I have submitted a problem statement, which, if satisfied would enable this attack. If you disagree with the statement, let's hear your rebuttal?
I claim it would be 'easy' to acquire a historical key because it would contain no funds, and thus be worthless to the seller. That seems fairly well justified rational behaviour to me.
edit: even if it's 'hard', the attack is still possible, regardless.