I could certainly do that (or anyone could if they wanted to sell their account), but wouldn't want to for a number of reasons. I don't desire to sell any of the Hive that is vested to the account, but want it moved to my personal wallet to power up instead of leaving it on my old partner's or having it all go to him for his bills. I could technically just use my personal infrastructure now to run a new witness node, or could have used the same infrastructure from that node that was in the top 20 to sign with a different account, but the point was to disable that shared witness that was known under a name and partnership we would no longer be supporting. Someone else could easily set up new infrastructure for much, much, much cheaper than the set of servers we were using for our top 20 spot. The account used to sign blocks as a witness doesn't actually matter, just the hardware and the software.
Selling the account privately to someone to keep running would be a really deceptive way to undermine the stake of people who voted for the account based on the belief they knew it was run by us, which I would simply never feel comfortable with. Also, I would not feel comfortable selling an account that spent so long in consensus even if it wasn't attached to a witness with a lot of governance votes, as it could be too easily used as a vector for something like a phishing attack. I'd much rather lay that account to rest alongside the witness and continue working solely under my own name and not having Jeff feel pressured to try to contribute more again.