Thanks to limited liability, it is very hard for companies to ever run the risk of negative value. Equity can be seen as a call option on liquidation value (plus dividends). So both options might be viable for rump Symantec: sale of assets / liquidation, or keep running it and hope for the best.
That's from a economics point of view.
From a more cynical point of view: shareholder capitalism is mostly a lie. Principal agent problems are real, and most companies are run for the benefit of management. And since managers are more important and can justify higher pay with an empire below them, the divestment will rarely happen. Especially if like for Yahoo (and perhaps Symantec) it would reveal in stark and undeniable terms that that very management of the parent company actually _subtracts_ value.
Some people did ingenious studies in this area: they checked how share prices reacted to unanticipated CEO deaths, like accidents. If management really served at the whim of shareholders, you'd expect that they'd have the best person they can afford. In practice, the share price goes up on CEO death as often as down. That means shareholders are often happier with the average expected next candidate for CEO than the one they currently have---but since they can't get rid of the incumbent that preference is only revealed on accidents.
(I couldn't find the studies quickly, but I found a quora discussion that might lead you there https://www.quora.com/Why-do-companies-stock-prices-rise-aft...)
