Here is the oft-cited history, in the salary-cap era, of how the losing team in the super bowl fares the next season:
1994: 49ers over Chargers (11-5, 3.6) -> 1995 Chargers: 9-7, 1.5 (#5 seed, lost WC)
1995: Cowboys over Steelers (11-5, 4.6) -> 1996 Steelers: 10-6, 5.2 (won AFCC, won WC, lost DIV)
1996: Packers over Patriots (11-5, 5.1) -> 1997 Patriots: 10-6, 5.3 (won AFCE, won WC, lost DIV 7-6)
1997: Broncos over Packers (13-3, 7.7) -> 1998 Packers: 11-5, 5.0 (#5 seed, lost WC)
1998: Broncos over Falcons (14-2, 10.0) -> 1999 Falcons: 5-11, -7.1
1999: Rams over Titans (13-3, 1.0) -> 2000 Titans: 13-3, 8.3 (won AFCC, lost DIV)
2000: Ravens over Giants (12-4, 2.4) -> 2001 Giants: 7-9, -1.8
2001: Patriots over Rams (14-2, 13.4) -> 2002 Rams: 7-9, -3.3
2002: Buccaneers over Raiders (11-5, 10.6) -> 2003 Raiders: 4-12, -5.5
2003: Patriots over Panthers (11-5, -0.9!) -> 2004 Panthers: 7-9, -0.7
2004: Patriots over Eagles (13-3, 5.6) -> 2005 Eagles: 6-10, -2.3
2005: Steelers over Seahawks (13-3, 9.1) -> 2006 Seahawks: 9-7, -3.6 (won NFCW, won WC, lost DIV)
2006: Colts over Bears (13-3, 7.9) -> 2007 Bears: 7-9, 1.2
2007: Giants over Patriots (16-0, 20.1) -> 2008 Patriots: 11-5, 3.9
2008: Steelers over Cardinals (9-7, -1.9!!) -> 2009 Cardinals: 10-6, -0.3 (won NFCW, won WC, lost DIV)
2009: Saints over Colts (14-2, 5.9) -> 2010 Colts: 10-6, 2.9 (won AFCS, lost WC)
2010: Packers over Steelers (12-4, 10.2) -> 2011 Steelers: 12-4, 5.3 (#5 seed, lost WC)
2011: Giants over Patriots (13-3, 9.3) -> 2012 Patriots: 12-4, 12.8 (won AFCE, won DIV, lost AFCCG)
2012: Ravens over 49ers (11-4-1, 10.2) -> 2013 49ers: 12-4, 10.1 (#5 seed, won WC, won DIV, lost NFCCG)
2013: Seahawks over Broncos (13-3, 11.4) -> 2014 Broncos: 12-4, 9.6 (won AFCW, lost DIV)
2014: Patriots over Seahawks (12-4, 9.5) -> 2015 Seahawks:
TBD
To dive into each team's situation and how players moved around, under- or over-performed the next year, etc would require a full article. But a couple things jump out at me just from this:
- Of the last 20 super bowl-losing teams, 12 (60%) made the playoffs. 8 times (40%) as division winner (3 of whom got byes), 4 times (20% as wild card)
- There was a stretch of 10 years (1998-2008) where the SB-losing team only made the playoffs twice the next year (2000 Titans, 2006 Seahawks). On average, they had 5.4 fewer wins (!) and 9.0 less SRS (point differential per game, with an SoS adjustment) that following year.
- But in the last 6 years (2008-2014), that team has made the playoffs every year. Those 6 teams have only averaged 2/3s less of a win per year than in their SB year, and 4.7 less SRS.
- Of the last 20 super bowl-losing teams, only 2 have made it as far as the Conference Championship games. But those two have occurred in the last 3 years (2012 Patriots, 2013 49ers). Those 2 were also the only ones to have a double-digit SRS in their follow-up year.
- Of those teams, only 2 have improved their win total over the previous year (2009 Cardinals, 2013 49ers), and only 2 others have stayed even in wins (2000 Titans, 2011 Steelers).
- Of those teams, only 2 meaningfully improved their team the next year: the 2000 Titans improved by more than a touchdown per game (+7.3), and the 2012 Patriots by more than a field goal (+3.5). 4 others improved by under 2 points, 7 of them declined by less than 5 points, but 5 saw double-digit declines.
- Of the teams who improved their SRS, or who declined by a field goal or less, ALL of them made the playoffs except the 2004 Panthers (who went from being an 11-5 team with a -0.9 to a 7-9 team with a -0.7). The other 10 made the playoffs, averaging no change in SRS and 1 less win.
What I interpret from this is that the era of super-parity, where teams can't sustain big success all that long, probably ended in 2008. The last 6 seasons have seen some of the biggest success for teams good enough to reach (but lose) the Super Bowl the previous year. But no team since the 1993 Buffalo Bills has returned to the Super Bowl after losing it. So I'd say the most likely outcome for the Seahawks next year is to narrowly make the playoffs but not make a big run. That's still a year most NFL fans would gladly take.