Actually the original premise wasn"t changed. It stayed the same. A ship out in space having a variety of adventures.
That is vastly over-simplfying things,
CE, and you know it.
"A ship out in space having a variety of adventures (as part of a vast organization with plenty of other ships to aid it and bases to support it)."
Is
very a different premise than:
"A ship out in space having a variety of adventures (with only the resources at hand, dependent entirely on the skill of its crew and the leadership/experience of its Captain).
The answer is that the original concept was the one used for the show. As was the case of TOS and the 5-year mission, it was never really going to be about how long the ship would stay in space or how far.
See above. Open up your mind a bit and contemplate just how differently things might've turned out if (for example) the
Enterprise encountered the Borg out in deep space all alone
without the ability to call up a battle group to intercept them or run back to the Federation for repairs afterwards.
They paid lip service to the idea again in Voyager, but again executive meddling kiboshed really showing us what that would have been like in a Trek context.
I mean the creative process. The writing of scripts and the development of the characters and the setting. The show wasn't mapped out.They had x number of scripts completed and x number in development when the show started. As plots were approved and the actors grew into their roles, I've a feeling the show changed, evolved if you will. Some early concepts fell by the wayside ( as it did with TOS). It's seemed that "Galactic politics" moved to the forefront and the "unknown" dropped back.
You could see little hints of it in some of the early scripts (mostly the pilot), that is true, but as early as the 2nd episode they were already clearly tacking back to the "classic" story model by having them encounter yet another Federation starship, which wouldn't've happened if they were using the original premise as
Enterprise would have been the only Federation starship in the area (literally).
I think the writers just like playing in that sandbox. Both Voyager and Enterprise left that sandbox and I think the writing suffered.
Both shows quickly scampered
back to that sandbox. Voyager tried to at least keep a formal semblance of holding to the premise of "one ship, alone", but never really implemented it, leading to the infamous "Voyager Reset Button" effect.
Enterprise did a better job initially of showing the premise, but by mid-late S2 it was back in full communication (after starting the process back in Season 1) with Starfleet and regularly doing all the "classic" stuff again.