A great deal depends, of course, on how close off the rack clothing is, for any given person, and whether it's worth the time and money to them to have a perfect fit. In many cases, it would improve more formal wardrobes, but I can't see a lot of people investing in tailoring for leisure wear.
I'm not sure tailoring can ever be completely computerized. Computer-assisted, sure, but fully silicon? Makes me think of this:
Yes, Sleeper was a comedy, and that scene was a parody of a stereotype. I simply couldn't let the chance to post it go by, though. ("Every line is a straight line; every line is a song cue.")
I actually do think it will be possible to do fully computerized tailoring at some point in the near future, though ramping it up to the point where it's common and inexpensive might be difficult. (One question is how expensive a measuring facility will be. Something like it does exist; the process of 3D photography, which involves laser imaging, is fairly mature, though the software to convert those images to clothing patterns and/or measurements will likely need further development.)
An alternative to computerized alteration of existing goods might well be 3D-printed clothes, though that, too, will have some trouble ramping up to high levels of production. (The materials themselves might be an issue; I don't know whether woven cloth can be printed effectively.)
How many people would be willing to pay enough to make this work?
Did something like this exist for everyone back before mass production, or were most clothes still to some general standard as individual piecework?
Some years ago I was flush and tried to ask around about getting custom clothes made, only to run into the problem with a lot of special interest stuff, that no one understands what you are talking about unless you already know the answers to the questions you are asking.
How much people are willing to pay is partly culturally shaped. I'm guessing that if tailored clothes were generally considered better, a good many people (a quarter of the population? a tenth? half?) would have fewer clothes made of more durable materials.
Consider the amount which is spent on attempting to lose weight-- the odds of getting what you want are much higher for tailoring.
It might be relevant that tailoring can be done on a cash basis.
A little history, which reminds me of Tom Wolfe's "drape vs. shape"-- I think clothing which closely follows the lines of the wearer's body might be no earlier than Elizabethan. This doesn't answer your general question, though.