The underlying premise is incorrect because carriers do vary their MCT's. However, MCT is simply a reflection of a carrier's willingness to assume the risk of a misconnect. It is not necessarily a Sensible Connect Time (SCT).

If the connection time worries you, then don't book it. DL has 10 non-stops/day LGA-DTW, so it's hardly necessary to fly via DCA.

But, if you do wish to book through DCA, 33 minutes is plenty of time. The USS (AAS?) gates are no more than a 2 minute walk, perhaps less, to the gates likely used for DTW. MCT is calculated from scheduled arrival to scheduled departure. If a flight is often late, presumably the carrier will adjust its scheduled arrival over a period of time. Needless to say, if a flight is delayed, that is when the MCT comes in handy as you are protected.

Nonetheless, it is the last flight of the night. Plan B is a hotel at DCA, so perhaps if you have something mission critical in Detroit the next day, you go back to the non-stops.