The Old Mixer wrote:
Another idea...we don't know that all of the Constitutions were doing the exact same type of mission as the Enterprise at the time of TOS. It could be that Kirk's 5-year deep space mission was the first successful attempt at such a mission without needing the other Constitutions and/or their crews to have all bitten the dust in a short time period.
Good call. A lot of people assume that 5-year missions are the standard, but that's unsupported, since we only had canonical evidence for one ship going on one 5-year mission -- and we barely even had that. After all, main-title narration doesn't really count as in-universe content, so the only real proof of the 5-year mission would be Kirk's "my five years out there" line in TMP and Icheb's reference to "Kirk's historic 5-year mission" in VGR: "Q2." Indeed, from those alone, we can't even say for sure whether the mission was meant to be 5 years or if it just happened to end up that way. (If we go by Roddenberry's implied interpretation that TOS was a dramatization after the fact, then the opening narration could be a reference to how long it ended up being rather than how long it was planned to be.)
Star Trek Into Darkness
is our first canonical evidence that a "5-year mission" is a specifically defined mission profile in Starfleet... yet at the same time, it's also canonical evidence that it's not the universal norm for Starfleet, that it's a distinct, even exceptional type of mission profile for a capital ship. Granted, it's in an alternate timeline, but it certainly supports the idea that the same could apply in the Prime timeline as well.