The Oracle of Omaha has spent close to $78 billion purchasing his favorite stock since July 2018.In a little over two weeks, one of Wall Street's most revered investors will step aside.Billionaire Warren Buffett has held the reins at Berkshire Hathaway (BRK.A +0.85%)(BRK.B +0.74%) for 60 years, and over that time has overseen a cumulative return in his company's Class A shares (BRK.A) of almost 5,942,000%, as of the closing bell on Dec. 10. The now 95-year-old Oracle of Omaha is set to retire at year's end ...