It does prompt the thought, that wasn't broached, as to why the Valiant's captain would have taken on such an unknowable risk, especially as he was not ordered to do so?
Dialogue suggests that forces of nature swept the Valiant not merely to the edges of the galaxy, but actually past the Barrier. The captain then only had to choose whether to try to get through the Barrier to reach home, or to stay outside the galaxy forever. An easy choice, I gather, especially as he quite possibly had no idea the Barrier posed any dangers (whatever damage it did to the Valiant on the way out might have been attributed to the original "magnetic storm").
Their presence in my vision was to provide backup assistance in evaluating this unknown realm through the provision of enhanced scanning and/or probing capabilities to be used prior to the entry being undertaken, so as to allow the most complete picture possible of what Enterprise was to encounter.
We now know that observing the passage of the point ship would have been futile, as the Barrier blocks communications and sensing. Would Starfleet have known that much? I must argue that it would have. Our heroes are not observably surprised by the existence of the Barrier - their mission appears to be to brave this previously carefully recorded phenomenon. This would be consistent with a gradual and extensive exploration effort wherein first the Barrier is stumbled upon, then probed, then found difficult to understand and impossible to study via sensors and probes, and the infrastructure then is established to allow a starship to attempt the next step in the meticulous process.
Can you provide any examples of that sort of time frame being devoted to any particular venture we are aware of in the TOS continuity?
Well, all of them - no research project in TOS is indicated to be short-term. Some are merely lifelong exercises for the researchers involved (say, Korby's studies), but others might equally well involve generations of researchers.
The Enterprise herself apparently took ages to "complete" - she's several decades old by the time of the TOS adventures, yet her engine designer is a youthful man still, and then the ship continues to evolve through TMP to the final ST2/3 form. Why halt any project at a couple of decades already?
First they are going home, presumably to have their descendants report on the suitability of the Milky Way for conquest. What relevance would Starfleet vessels being able to weather the challenges of the Barrier have on such plans?
Every possible relevance, of course. The heroes would hope to stop the voyage home, and two ways to do that would be to a) tell the Kelvans that the ship is not up to the task, and b) not tell them that. That the issue doesn't arise and the ship still survives means our heroes made neither of the above efforts...
Timo Saloniemi