You are confusing copyright and trademark.
MM is of course protected by both at present.
A trademark is in perpetuity as long as it is actively in use.
A copyright was intended to be for “a limited time.” I seriously doubt the Founders intended that to mean for the life of the author plus 70 years.
You have a perfect right to argue that copyright SHOULD be for a very long time as at present. I don’t think you can logically argue that this is what the Founders intended.
While limiting the use of innovations can be said to be an impediment to progress, I don't see how a similar argument can be made for the exploitation of an image or character someone else created - I don't see that as progress.
Patents for innovations are for 20 years. Copyright on stories, places, characters and images created by an author are for the life of the author plus 70 years - that is PERFECTLY reasonable in my opinion.
In the time of the founders the timeframe was 14 years for useful technologies and such - I don't think 20 years is significantly different than 14 as to making an argument that 14 years is in line with what the founders intended but 20 is far beyond what they intended. So I CAN logically make the argument that 20 years is well in line with what the founders intended, because 14 is what they granted in 1790.