The idea was that maybe they could upscale it to movie size with lots of spectacle, but even then the effects house was being pushed against it's limits. Hence, alas, the memory wall was abandoned...
That's effects houses since it was Future General (Trumbull) and Apogee (Dykstra). Also, Trumbull was probably wise to suggest the change even if it wasn't for the effects budget. The sequence as scripted is slow and features a silly Raquel Welch moment. It's better that the journey is Spock's alone, and that it's over in a couple of minutes flat.
Pretty much, yeah.
Plus, look at the difference between those space suits - the pre-Trumbull one is just awful.