I don't think Hoke was much better, I just think Maxwell is that bad. I don't expect that trend to continue indefinitely, but I don't expect Michigan to have a great defense this year either.
Harbaugh is miles better than Hoke as a coach. I don't know if he's really that much better than RichRod, who has won a lot of games at other schools, but RichRod was a bad fit from the get-go. I think Harbaugh will get the defense playing more sound, but that may not be enough with so many players that just aren't nearly as good as their recruit ranking suggested. And against MSU, a team with an NFL caliber QB and a really good O-line, I just don't think Harbaugh alone is going to be enough to turn that defense into a great unit.
That said, football has a funny scoring system. Is scoring 28 instead of 35 all that big of a difference when it's really just one score? Similarly, if MSU drops 42 on UM, I don't think it means UM will have necessarily taken a step back on defense. MSU might just have an elite offense this year, and it could even be a shootout on both sides.