Tyson's career was clearly already on a downslope when he went away to prison. He may have won the several fights he had after the Douglas fight, but they were not impressive.
What made Tyson unbeatable during the late 80s was not his power, it was power in combination with great head movement, hand/footspeed, and the ability to throw a jab and throw tight combinations.
Yes, he was the hardest puncher in the division, but Kevin Rooney also taught the importance of technical precision.
During the couple years right before prison, Tyson stopped moving his head, stopped working his jab, stopped concentrating on fluid combinations,stopped training properly, etc. His technique was completely deconstructing to the point where he was doing nothing but trying to knock guys out with one punch, his defense and movement were being completely ignored.

He was a rudderless ship without D'Amato/Rooney and the effects were evident. All prison did was delay his inevitable downfall. If he never went to prison, he just would've gotten beaten up by a young Holyfield and young Lennox Lewis, instead of getting beaten up by an old Holyfeld and old Lewis after prison.