Yeah, the people in the safe seats aren't beholden to anyone (and not entirely in a good way --- this freedom tends to make them irrelevant in the long run).

I don't agree that their pushing of issues brings those issues into the mainstream. The major breakthroughs of my lifetime have either come from rigid party discipline (Obamacare), or from sea-changes in public opinion over individual, personal experiences (LGBT rights/marriage equality, weed legalization). The "left" has been on the proper side of history on all these issues, but at best they were irrelevant to actually making it happen or even building the early support.