Say TS, due to the writer's strike, COULD have done Indiana Jones. By contract, yes, he had to come back and at least start MPI. But if the movie had such great success with TS as it did with HF, would he have stayed with MPI for seven, much less eight, years? Had he done the first Indy movie, I think MPI would NOT have been made the way it was. And the viewing public would have been robbed of MPI for 7-8 years. We here all know what we would have missed -- the characters, the loyalty, the adventure, the humor, a little snippiness, etc., etc.
I believe that TS's MPI contract was for 7 years. He ultimately decided to honor his contract to do MPI rather than "Raiders of the Lost Ark."
As Golf mentioned, it turns out that there was a writer's strike which would have allowed TS to do the movie and return in time to do the first season on MPI. However, by that time HF had already been cast for the part. I have no doubt that if TS had actually made the movie and it turned out to be the big hit that it was that he would have honored his contract and returned to do MPI for the seven years at least. The question is whether he would have been able to do the next two movies?
I still say he would have honored his contract to Universal. Then they would have had to cast a different actor (HF) in the Raiders
sequels. It worked for the movies based on Tom Clancy's novels. Alec Baldwin played Jack Ryan in "The Hunt For Red October"
and Harrison Ford was cast to play Jack Ryan in "Patriot Games," "Clear and Present Danger."
Tom Selleck seems to be a very honorable guy and a man of high character, and his character traits come across in Magnum, which is one of the many reasons why MPI was such as big success.