Obama won off his campaign. That suggests that enough people agreed with his platform to vote for him. He didn't win additional votes by shifting to the center: he shifted to the center after already having received said votes.
And polarization of the voter base helps that argument: people want real, radical change. On both sides of the political spectrum. The right is having their demands heard through the rise of Trump's fascist tendencies. The Democrats need to notice that the "moderate" Rebuplicans were more willing to vote for a fascist than for a moderate Democrat, and re-evaluate their platform.
But the change many people want is not immediately achievable, at least not without a period of intense suffering as a result of drastic upheaval (read: war). It defies basic laws of humanity.
You seem to suggest that politicians like Obama failed to achieve all their campaign promises, even though they could have. I suggest that they could not have. that it wasn't a bait and switch, but rather laying out an aspirational vision and then trying to achieve as much of that vision as reality allows for given the many needs and competing interests that co-exist in the world at any given time.
And polarization of the voter base helps that argument: people want real, radical change. On both sides of the political spectrum. The right is having their demands heard through the rise of Trump's fascist tendencies. The Democrats need to notice that the "moderate" Rebuplicans were more willing to vote for a fascist than for a moderate Democrat, and re-evaluate their platform.