2 Billion is NOTHING. It's a single acquisition. Sure they could lose a minor amount of money for years with the cash on hand but without growth the stock tanks and they become an acquisition target. The danger isn't SUN going out of business tomorrow it's that they will be bought out by someone or that they will slowly bleed themselves dry in much the same manner a certain company by the name of Santa Cruz Operation (The original one) or SGI did.
UNIX, the proprietary version, is dead. It was dieing when SGI and SCO got into trouble and it's long since dead. The only people still using it are the ones that were already using it. Based on trends most of them will switch to Linux by the end of the next decade as they upgrade. The proof of this is to look at IBM. AIX for the legacy customers, all systems can run Linux so the customers can switch to Linux as they convert applications from AIX. The proof is also in NASDAQ switching to an entire Linux backend a few years ago, a contract I might add that SUN used to hold and I believe IBM now holds. Legacy clients are switching, and eventually all of them will switch because the benefits of being vendor neutral are major.