I absolutely adore The Motion Picture, but a half-finished and untested refitted Enterprise being the only ship available to intercept V'Ger is nonsense.
Lol. Don't come for TMP bro! Only STARSHIP in 'interception range'. Don't forget, V'Ger was travelling really, really fast so any ships behind it would be unable to catch up in the time available.
Plus Earth has automatic defences in the Solar System. It doesn't need starships for defence.
The reason why I can give TMP a pass and not Trek09 is because the writers sat down and calculated travel times (it was likely hadn't decided exactly how far away Klingon space was yet but they also weren't specific about how far away Epsilon IX was from Klingon space so they could fudge the times there).
In Trek09, they overtake Nero at warp 4, so any ship travelling at Warp 7 or more could probably have caught up to him. If they had known his ship was that slow, it would have been better to send an emergency code via subspace relay to an undamaged ship which could reach the fleet sooner and then for the fleet to head back to Earth at emergency warp. But his ship isn't that slow because of how quickly he reaches Vulcan after leaving Klingon space. This one doesn't really make sense because it contradicts itself. Why DOES he travel so slowly to Earth? He's like the little dude aiming guns at Yavin in Star Wars.
TWoK doesn't really make sense at the end because they are fleeing at sub-light and a nebula would take years to reach.
TFF doesn't make sense because it would take years to get to the super massive black hole at the centre of the galaxy. They could have used some other thing similar to the Necrit Expanse instead.
STVI doesn't really make sense because the travel times are condensed to a really short time frame and the peace conference would realistically take months to organise.
So yeah, lots of silly stuff but TMP is not so implausible. They should have mentioned defences in the outer solar system though.