MSP/DTW flights are some of the more consistently boarded flights by elite level.
With that many elites NW will frequently board by First Class, Platinum, Gold, Silver, then continue with boarding. Of course, that was pre-Deltification and Zone boarding. It's still possible they'll board by elite level and then zones 3 & 4.
There are a ton of threads on the old NW forum about upgrades between MSP & DTW. From my reading I'd say it's likely 50% or more of plats don't even get upgraded on this route.
Your best bet is to pick an off time and to pick the larger aircraft (757). NW often flights two MSP/DTW flights back-to-back (e.g. 5 minutes apart) and one will be a small aircraft like an A319 or DC9 while the other is a 757.
It's a very short flight so enjoy your exit row if you were able to book it. The 1,000 Plat Guarantee is gone so you won't even get a consolation prize anymore.
-RM