As MS is even a stock company it is even unclear to me, how taxing can help. Even if Bill Gates would have to sell shares every year just to pay for a wealth tax, the people who own the shares afterwards would still looking for monopoly like returns.
If Murdoch sells newscorp shares to some institutional investors, which hold as well other stocks, a business friendly press might still be their interest, despite no single person owns billions.

Divestiture is the only thing which could help, in both cases.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers