It gets brought up in debates all the time. One of Klobuchar’s favorite things to say is that Medicare for All isn’t “real,” and not even Democrats support it.
The reason that it isn’t a compelling way to attack Sanders for having an agenda is that no Democrat ever gets anything done, if you’re going to apply that logic to it. If it’s a serious mark against Sanders that he won’t accomplish “it,” where it is everything he’s talking about in February, then you might as well not run a candidate at all, because it’s 2020 and the Democrats have at this point essentially given Republicans a heckler’s veto on any action that they might take, no matter how incremental or moderate.
No Democrat is going to walk into office and just start checking off boxes without being opposed at every turn. As with many other issues here, everyone seems to acknowledge and understand that when they’re talking about anyone but Sanders. Pete Buttigieg isn’t going to go in front of Congress and hear “sure, Pete, let’s work together on this, since you seem pretty presidential and unthreatening to our interests!” If it’s supposed to be a meaningful condemnation of Sanders’ agenda that people are gonna try to stop him, you have to demonstrate that Sanders, in being opposed in that way, is meaningfully different from anyone else. And it certainly isn’t true that other candidates are going to walk in in a better negotiating position, having already internalized a lot of the Republicans’ talking points.
Now, is it a compelling thing to point out that Democrats will oppose Sanders’ agenda. Yes, absolutely. But, like, they should stop doing that, not point to it as a badge of honor.