The prospect of Satoshi being dead wouldn't cause panic on the market.  He didn't partake in Bitcoin development after mid-2010 and stopped posting on the forum around the end of the year, so him dying several years later in 2015 wouldn't affect the market at all.

Regardless, iamnotback is clever enough to realise that his belief that John Nash was Satoshi won't make everyone else believe it.  There was an article on bitcoinpricelive and an an article on CoinDesk arguing that this was a likely possibility years ago, but people didn't jump on the train because the "circumstantial evidence" provided is not, in fact, quite strong enough to prove that he was Satoshi and it can't actually be reasonably proven without Nash himself signing a message with the Genesis block (which he can't - he's dead).