It's interesting when you look back at all the NFL dynasties -- Packers of the 60s, Dolphins of the early 70s, Steelers of the 70s, Niners of the 80s, Cowboys of the early 90s, Broncos of the late 90s -- NONE of them lost their last Super Bowl appearance. And really, that seems to separate the dynasties from the merely great teams.
The Cowboys in the 70s lost every other Super Bowl they appeared in, so they're not a dynasty. The Redskins of the 80s and the early 90s weren't a dynasty either -- their championships were spread out over 10 seasons, and included three different starting quarterbacks. In the last 12 years, the Packers and the Rams lost their follow-up
Super Bowls, though the Rams' appearances were separated by a year.
If they'd been able to win back-to-back titles or even two in three years, Green Bay and St. Louis would have had legitimate claims to the title of "dynasty."
Basically, these Patriots have the chance to be the first dynasty -- and they already are a dynasty -- to lose a Super Bowl. They probably won't, because of experience, but there's a first time for everything.
Mind you, the Pats can always return to the Super Bowl in the next few years, and start a new chapter. But no dynastic NFL team has ever lost a Super Bowl and then won another title. The Dolphins in the 70s lost their first appearance, but then won two in a row. What's this say about dynasties? It says they're always capped with a W -- to date, anyway.