sanders has not changed the democratic party, and is not the party's future. rather, what he's done is demonstrate that the democrats are a dead-end for substantive change, and created a third party in american politics. that third party is not yet organized, but if it is not organized by sanders or stein then it will organize itself.
so, no - sanders is not the future of the democratic party. the democrats put themselves in an opposite direction back in the early 90s, with clinton/gore. rather, he's representative of the final fracture point that the clinton/gore/obama direction has pushed the party in.
the democratic party as we know it will not survive the belligerence of a hillary clinton presidency, or even the belligerence of a hillary clinton candidacy.
http://www.vox.com/2016/6/9/11867810/bernie-sanders-obama-future-democrats
but, let's be clear. the democratic party has been fracturing for twenty years. what happened around obama was....he wasn't what people thought he was. people shouldn't have thought he was what they thought he was. and, a lot of people never realized he wasn't what they thought he was - still haven't, never will. they've still got this weird meme in their head. if that's the future of the party, it's a brave new world, indeed.
so, the better analysis is in terms of fracture points rather than approval ratings. and, what we're seeing this cycle is the consequence of a process, not the result of a personality.
if you want to blame somebody, the people responsible are the clintons.