Someone once said that it's a matter of comparing one set of fake looking f/x with another more contemporary set of fake looking f/x. And it is. Neither the original nor the cgi f/x look truly realistic, but what really matters is what an individual finds more convincing.
For me the original f/x, flawed as they are in some respects, are more convincing in conveying the ship's size and sense of majesty. I find the new f/x, while quite interesting in many respects, generally fail in conveying the ship's sense of size and majesty.
The original f/x were state-of-the-art for television
at the time but not for feature film. However, it wasn't a huge gap since the main difference was time and money. Given sufficient time and money TOS' f/x could have been on the level of 2001: A Space Odyssey
and Forbidden Planet
and Planet Of The Apes.
Films like Alien
and Star Trek - The Motion Picture
aren't that far removed from what was being done a decade or so earlier. But then it wouldn't have been possible because a TV series production doesn't have the luxury of a feature film's time and money and resources.
The situation is still pretty much the same today. In any era really you can lavish a feature film with awesome f/x because you're making only one picture.
You simply can't afford to do that with series television.
The TOS-R f/x suffer from the same issue of not enough time and money available. It really comes down to time = money and the TOS-R guys simply didn't have enough time to do better.
That said I still think a bigger issue I have with TOS-R is the aesthetic of the new f/x. To me they don't look like what could have been done under the best of conditions
back when Star Trek
was in production. If I were to approach enhancing the original f/x I wouldn't be asking, "What can we do that they couldn't with today's
resources?" I'd be asking, "What could they have done if they had had access (and sufficient time allotted) to the best resources available at the time?"
The difference is you're now trying to put yourself into the minds and perspective of the f/x artists of the time and seeing it through their eyes rather than a perspective coloured by forty years of advances in visual f/x. The end result means your new enhanced f/x should look more aesthetically consistent with the remaining live-action footage.
This is where I think TOS-R generally blew it.