How does the upper class benefit from being leftist?
Ask them.
Are you disputing the the upper class, the rich, are not largely made of Leftists or people sympathetic to Leftists causes and their agenda?
They want the destruction of the middle class because you can't enforce your dictates on the middle class, the middle class makes their own money and thinks for themselves. The lower classes are easily led by controlling their money supply, give them welfare money, housing and health care and you can control them.
They want a society where the elite are unchallenged at the pinnacle and you can't have that with a large middle class controlling most of the resources. Think of feudalism, a few royal and noble families living in extravagance while peasants lived in filth. All totalitarian systems take that same form, communism is the same with the rulers living in extreme wealth while everyone else suffers for "the good of the people".
How does the upper class benefit from that? They get almost unlimited wealth and power, that's how they benefit.