Millions of people would suddenly have their support contracts voided. Thousands of I/T professionals would get into work the next day and not be able to get parts replaced under warranty.
And the next step for those millions of people and thousands of IT professionals? Call their lawyers. It's a good thing Apple would have billions left over because they'd need it to pay for legal representation and class action verdicts. In practice, of course, buying and "shutting down" a company doesn't end your liabilities (financial or legal) -- just ask JP Morgan Chase about Washington Mutual or Bear Stearns -- and Apple would be on the hook to see through those service contracts (which would certainly be cheaper than voiding them and dealing with the fallout). In practice "shutting down" Dell would take years as those loose ends are wound up.
And yeah, it would leave a vacuum in the market but that would be quickly filled by HP, Lenovo, and others.