I think a better comparison for any "omg Michigan is competitive" thing is Indiana - How can we say Dan Coats is safe in Indiana in 2010 when Obama just won in 2008!
I doubt anyone ever thought that Coats could be in trouble in 2010, and MI is still much more competitive than IN. But yeah, while Stabenow is basically a generic liberal Democrat and not really a good fit for her state, beating her won't be easy. I wouldn't rule out an upset if Trump is popular, but I'm not sure which Republican could actually make it a race. I'd also rate it Likely D at this point, but definitely not Safe.