2001 is brilliant. And, fond of it as I am, TMP is badly paced with a weak script.
I don't think it's fair to blame the distributors. Paramount block booked the film for that date and what were the theaters supposed to do if Paramount delayed it?
The TMP producers and Wise are both to blame for not getting the film done to their satisfaction by the contracted date. As a producer myself, I know that when you see a looming deadline and potential problems meeting it, you reassess. There's no reason Wise couldn't have tightened up the edit of the live action stuff while waiting for the effects shots to come in. And re the effects, the responsible thing to have done would have been to trim down the number of shots and concentrate on a few really memorable images as opposed to a whole lot of shots that aren't necessary even if you want them. For instance, we don't need four minutes of flying over V'ger, we just need a handful of OMG THAT THING IS MONSTROUS shots to sell the hopelessness of the Enterprise's mission.