Peyton stayed his senior year because the Jets had the #1 pick in '97. Eli after '03 was projected to go behind Palmer and Leftwich so he stayed another year. Both did a good job of sugar coating their decision though. Get a degree. Blah, blah, blah.
I agree that it's not likely Arch declares for the draft next year but if the Saints have the #1 pick it might be too hard to pass up. The Giants and Colts could be in play too. You never know what team's picking first in 2027. He's actually the betting favorite at +200 to go #1 next year. It'll be an interesting story to follow. That's for sure.