The second they decided to remaster TOS for high def, redoing the visual effects shots was a given. The original effects shots often times don't fully hold up under a standard def tv (which is almost twice the resolution of a high end set back in the 60's), there's no way they can hold up on an HDTV.
The only solution was CGI, both from a time and money perspective, and because of the simple fact that the original effects element shots, i.e., the footage of the models on the effects stage, starfield shots, etc., have almost certainly all been destroyed by now (that being said, I wouldn't be a bit surprised if startrekhistory.com comes up with a few feet of footage, but the point remains). And, sorry, but there is no way that they're gonna drag the original model out of the Smithsonian and reshoot all those effects scenes.
The only solution was CGI, both from a time and money perspective, and because of the simple fact that the original effects element shots, i.e., the footage of the models on the effects stage, starfield shots, etc., have almost certainly all been destroyed by now (that being said, I wouldn't be a bit surprised if startrekhistory.com comes up with a few feet of footage, but the point remains). And, sorry, but there is no way that they're gonna drag the original model out of the Smithsonian and reshoot all those effects scenes.