Has she? She may have given in on international trade and on Wall Street regulations, but when it comes to single-payer and other issues, she has (more or less) held her ground.
I don't get why singlepayer is so necessary for a Democratic nominee, or even being used as a measuring stick for how left you are (I see it used in this fashion quite a bit). For the time being, it will not pass Congress, even if we had a supermajority in the Senate and control of the House. Democrats will not risk losing control over healthcare reform for the 3rd time. They will put their own interests ahead of that this time around, should it ever come to that.
I could see universal health care (not sure why everyone's so fascinated with single-payer, there's literally dozens of ways to implement UHC and single-payer's only one) becoming a hot topic again in a decade. A lot of the Democratic mover-and-shakers that had to move mountains just to get the watered-down ACA passed probably aren't too thrilled by the idea of opening that can of worms again in the near future, though.
