In the following example, say I had a majority of stake at block A. Then at B0, C0 all the way up to head block H0, I sell off my majority of stake, understanding that there is an upper limit on stake transfer.
A - B0 - C0 - D0 - H0
\ B1 - C1 - D1 - H1
Then, I create a double spend of my stake using the historical private keys, which I still own, thus sending it to myself in a fork continuing fork B1 - C1 - D1 that I create and approve myself (majority stakeholder).
This is not a 50% attack, because I don't own the value held in the private keys anymore, the cost for doing this is nothing.
Why doesn't the network just accept this fork as canon, given that its the same length (or longer if I chose) than the genuine fork?
Any single person owning (or colluding with) a quorum (majority) stake is a problem because that person (or colluders) can rewrite the chain as they choose. This situation is essentially the same as an adversary owning a quorum stake which is the limit of the protocol.