I get the impression that Stabenow's been around long enough to be considered a bacon bringer, so she's simply put up with. She's certainly closer to her state's interests than Levin was, but both were and are kept around for the same reason.
Neither is a particularly effective bacon bringer, but you are correct--tenure is a serious strength. If Stabenow wins in 2018, it'll become increasingly difficult to unseat her going forward.
As long as she's voting against automobile carbon emissions and fuel efficiency standards, the bigwigs in Metro Detroit will be okay with sending her checks.