I don't know what Arch will do. I do know the Mannings are very aware of Brand and Legacy. Both Eli and Peyton could have left after their Jr Years and probably been number 1 picks but chose to stay 4 years in College. A college player, especially at a blue blood like UT, undoubtedly has a bigger brand and legacy if they stay all 4 years then if they leave after 3. Whether or not that difference in legacy/brand between 3 years and 4 years is worth it to the Mannings I have no idea.
But without a doubt Vince's legacy would have been way bigger if he had come back his Sr. year and won the heisman and a 2nd title, who knows he might have gotten drafter by a better team also. Arch certainly doesn't have the financial need to get to the NFL as soon as possible and playing the QB position, he shouldn't have the usage concern before he gets to the league that say a RB has.
Again, no idea what Arch and them will do. I can say if I was sitting on the financial position the Mannings enjoy and you told me I could spend another year at UT or go to the NFL......I'd stick around another year in Austin.