Here is how BYU has done in the last 10 seasons in which they have had a returning senior QB at the helm, and which major accolades the QB earned that season:
2009 - Hall, 11-2 record, #2 in conference, #12 national ranking in final polls; Hall was voted 2nd-team all conference
2006 - Beck, 11-2, #1, #16; conference offensive player of the year (POY)
2001 - Doman, 12-2, #1, #25; conf. off. POY
1999 - Feterik, 8-4, #1, unranked; 1st team all-conf.
1996 - Sarkisian, 14-1, #1, #5; conf. off. POY
1991 - Detmer, 8-3-2, #1, #23; conf. off. POY, #3 Heisman voting, won Davey O'Brien award
1985 - Bosco, 11-3, #1, #16; 2nd team all-conf, #3 in Heisman voting
1983 - Young, 11-1, #1, #7; conf. off. POY, #2 Heisman voting, won Davey O'Brien award
1981 - McMahon, 11-2, #1, #13; conf. off. POY, #3 Heisman voting, won Davey O'Brien award
1979 - Wilson, 11-1, #1, #13; conf. off. POY
- In total, BYU's combined record during these seasons was 108-21-2, for an .837 winning percentage
- BYU won their conference title in 9 of the 10 seasons, and finished 2nd the other one
- We won at least 11 games in 8 of those seasons
- BYU finished nationally ranked in 9 of the 10 seasons, and 7 of those final rankings were in the Top 16 (it is worth noting that even in the one year they didn't finish ranked, Feterik's senior year, BYU was ranked in the Top 15 in November, before Staley was injured and the wheels came off)
- Only in one of those seasons (1999) did BYU lose to Utah
- In 8 of those seasons, our senior QB was voted 1st team all-conference, and in 2 of them he was 2nd team all-conference
- In 7 of those seasons, the BYU QB was voted conference offensive player of the year, and in 4 of those seasons, our QB finished in the Top 3 in the Heisman voting
During Feterik's senior year, the team was not spectacular, but still won the conference and was ranked most of the season, before late-season injuries to key players (notably Luke Staley) did them in.
Not sure how Riley will do this year, but history suggests that we should be in decent shape.
Edit: the original version of this post included 1988, which I mistakenly thought was Sean Covey's senior year. I have since corrected this error, since Covey was a junior in 1988, and backed up Detmer as a senior. Thanks to Yz1 for noticing the mistake.