Obama also presided over the greatest expansion of wealth inequality since the Great Depression era. He's also instrumental in the TPP. Obama is a pretty blatant corporatist and his accomplishments elsewhere don't give him a free pass on this.
So why exactly is being a "corporatist" (and I put it in quotes because clearly you define everyone who isn't of a "soak the corps" mindset as a corporatist) a bad thing?
Last I checked the United States still primarily functions via a capitalist economy. Corporations tend to play a key role in how that entire model works. I mean, you're posting on a website funded by corporate advertising via a web browser made by a corporation, running on hardware designed and manufactured by at least one corporation, using an internet connection provided by another corporation.
Welcome to the reality of the first world, corporations are powerful multi-national entities that have significant pull within the political process.
The problem, as it pertains to the American form of government at least which was inherently designed to work with a capitalist economic model, is the role corporations are given to play within the political system.
A bad corporate actor would be the Koch Industries' efforts to built the trans-pacific pipeline. They're funding politicians to ignore the ecological/environmental due diligence and vote against their constituency to support a single item benefiting primarily a single corporation.
But on the opposite end of the spectrum (for this one issue) you have Apple putting forward all their legal muscle to challenge an FBI request to break private data encryption as a slippery slope.
The later needs to end. The former is the idealized outcome of this model. If Apple wins it will be the single biggest curtailing of federal investigative authority we've seen in decades. All thanks to a corporation pushing back.
The average person can't push back against something as large as the federal government on any issue, at least not substantively. And not every issue can be solved simply through grass roots political movements, at least not as long as most of the electorate only tunes in once every four years. Corporations, with the right set of rules, are a benefit to the nation not a hindrance. Unfortunately Sanders' suggestion amounts to scapegoating them for economic inequality instead of the real root of the problem, wealth inheritance and the power that buys individuals (not corporations) already at the top.
The United States isn't just waiting for the right person to come along and lead it into the socialist promised land. That is not what the vast majority of Americans have any interest in. That is Sanders' ultimate problem. He's really popular with a segment of the extreme left base, just like Trump is really popular with a segment of the extreme right. A President needs to serve both sides, even when something they're ideologically opposed comes along with sound logic and clear merits, no matter how distasteful it might seem to be.
One of Obama's more iconic lines from his first POTUS run was stating an unwillingness to sacrifice the good in search of the great. That is an absolutely critical viewpoint required of the POTUS and across the entire primary season field Hillary Clinton is the only person who offers that.