Despite this I am perfectly fine, in my own fandom, assuming that there may have been other five year missions out there.
Sure, there could've been. But isn't it logical to assume they weren't the
only mission profile? A 5-year tour of deep-space exploration and patrol is surely just one of the many different kinds of missions that Starfleet vessels would need to undertake. There would undoubtedly be mission profiles of shorter duration, or possibly open-ended ones with no set duration. There would have to be. Sending a ship out for five whole years is a huge undertaking -- heck, that's why they called the show
Star Trek, because a trek is a really massive, extended journey or operation. It only stands to reason that there are other, less extreme types of starship mission out there.
Indeed, we've seen the
Enterprise on several. According to
The Making of Star Trek, Kirk had already captained the ship for four years as of the second season. Since I doubt they intended to end the show after three years, given the choice, that means that he was its captain for at least a year or two
before the 5YM started. And this is consistent with "Where No Man Has Gone Before." A dedicated mission to probe to the edge of the galaxy and beyond is something that would take months, just considering travel time alone. It's unlikely to be just one more assignment within a generalized 5-year survey and patrol tour; it stands apart as a separate "trek" of its own. So it looks as if the
Enterprise under Kirk was assigned to one or more short-term missions
before it began its dedicated 5-year tour. (Again, aligning with the Kelvin Timeline, where Kirk began his 5-year mission two years after being given command.)
After TOS, we don't know exactly what happened between TMP and TWOK, but we do know that by TWOK, the
Enterprise was serving in an apparently open-ended capacity as a training vessel -- again proving the existence of starship mission profiles other than 5-year deep-survey tours. And the E-A seemed to be assigned to individual missions as needed. The narrations in the movies referred to the ship's "ongoing mission," not defined by a specific duration.
Many of the writers made this very assumption in the 1980s novels when people were not sure when the chronological placement of STMP would be. The assumption was Kirk commanded two five year missions, at least with some of those novels.
No novel ever actually said that. That's an interpretation some fans (including myself) have made after the fact. There were some novels that established the pre-TMP period lasted several years longer than you'd think, but they never explicitly came out and said that the ship was specifically on a "second five-year mission," rather than the mission just being open-ended. That's just a later fan theory to make sense of those anomalous time references.
So, the idea of other FYM is out there, regardless.
Yes, I know it is, which is exactly why I think it's important to offer a countervailing view. Fandom tends to jump to the conclusion that
all missions must be 5-year missions, and I'm pointing out the flaws in that assumption. Certainly 5-year surveys could be
one standard mission profile, but it is simply nonsensical to assume that
every single starship mission ever has to be 5 years in duration, both from a common-sense standpoint and given that we have explicit evidence to the contrary. It's a failure of imagination to ignore other possible mission lengths and types.
What I've suggested in two or three of my novels is that five years is the recommended
maximum tour duration for a
Constitution-class vessel before it needs to be called in for refitting and resupply. Connies could be sent on missions of various different durations, but five years is the longest they can reasonably withstand, so that's why the longest-term mission profile, an extended tour of deep-space exploration, patrol, and colony support, is limited to a maximum of five years, though it could always be cut short for various reasons (certainly we saw several Connies' missions cut violently short in TOS). And it could even be extended beyond the recommended 5 years if the ship and crew were in condition to handle it (my books have Kirk pushing for an extension beyond 5 years but not getting it). That fits with the "Mark of Gideon" line about the ship having a 5-year supply of food aboard. And I think it works as a good compromise position -- the 5-year mission profile is a standard benchmark for Connies, but it's more a recommendation than an absolute guarantee, and it's just one of many possible mission types out there.