"It’s becoming increasingly obvious that Democrats can only win by moving to the left, and just as obvious that those in control of the Party would rather retain power than win victories."
As usual, I'm mostly with you - but let's challenge your analysis and push it further some.
First, I think economics has influenced the shift right:
On the one hand, the post-industrial, 'service' economy has meant: fewer unions and, thus, a weakened labor lobby; fewer unionized workers, and, thus, less 'class consciousness' based on collective work conditions and union leadership.
On the other hand, conversely, the post industrial economy has meant more post-industrial workers, meaning a larger 'atomized' labor force - a temporary, overworked labor force with less of a 'labor vs. capital' consciousness, less connection to fellow workers, and less of a union voice to support a political point of view.
2) At the same time, this does NOT mean that the right liberals that have taken over the Democratic Party are correct in their 'triangulating' analysis - the view that the electorate has shifted right so that, for Democrats to win, they must shift right, to the center too.
Rather, I would argue that the political class - mainly rich people - trend to the right, and were only prevented from doing so by the New Deal economic/political period. In other words - right liberals are not just pragmatic liberals; they are right wing liberals, previously checked by a more active left liberal electorate.
What this means in practice is that right liberals have never been open to the possibilities of increasing political power by engaging their left flank on issues - from Wall Street to healthcare to political party reform - but, as rich, right-trending people, have tended to explain Democratic losses and seek solutions by moving right.
The Sanders campaign shows just how much progressive populist muscle power they have ignored - and stiff-armed - that could have driven any number of more progressive Democratic campaigns - based on issues that many Americans support, including parts of the right wing.
Final upshot of this long post? First, they don't just want to lose - right liberals have deluded themselves with their right liberal ideology. Second, right liberals, deluded and right wing as they are, will never change their minds, never share power willingly. They will share power only when they are forced to.
What forcing them to share power means in practice - takeover vs. third party - becomes the basic question.