APS was the answer to questions that the film industry kept asking for decades, it just arrived late.
First of all, it is fool-proof. You just insert a plastic cartridge inside the camera and close the door. Just like with 126 or 110. This is very important if you want to increase film sales. Many persons, my parents included, would just have the roll loaded in the camera by the shop-keeper. And when they have finished the roll, they keep all the camera to the shop. Imagine how much that can damage film sales.
Also, the APS format allows the use to change, in a perfectly easy and error-free way, a roll mid-way. You can switch from ISO 100 to ISO 800 and back with almost the same ease as with digital. The camera IIRC remembers where the cassette was extracted and when you put it again in the camera it aligns the film with the first useful frame, no error, no waste.
The APS format gives the user the negative (or possibly slides) back inside the cartridge, where they can be perfectly preserved for decades. No more stripes of 4 when you have sleeves of 6, no more dust and scratches on old negatives. You never touch negatives or slides. You don't need cutting and touching to frame slides (you don't frame slides).
It is possible to devise, and it was done, APS film scanners where you just put the cartridge in it, and the scanner will scan the entire roll in sequence, without need of assistance. Now you either deal with - in the best case - stripes of 6 frames, or you scan mounted slides one by one. It goes without saying that with mounted slides you have dust problems that are possibly very reduced with APS cassettes (the cartridge is light-tight and so also pretty much dust-tight).
Is possible to devise a slide projector that is programmable for each cartridge so that the scanner knows which slides it has to skip during projection. No more frames, no more putting all the frames inside carriages with the occasional slide upside-down, and then putting them again inside the plastic box. And imagine a slide projection without all the click-clack-stomp between frames (and without stuck slides).
I think APS was basically a great idea, with some flaws IMO:
1) they chose a format smaller than 24 x 36. That condemned the format to the casual user niche. Professionals need quality. Advanced amateurs mostly tend to copy the behaviour of professionals or what they perceive as such. By choosing a small film surface, they confined the APS format to the typical Instamatic or 100 or photodisc user. If a camera wants to exploit all the possibilities of the APS it has to cost something more than the bare minimum. Slides, where the APS format could have had a very bright future, were entirely missed because slides are for advanced amateurs looking for quality, not for the Instamatic market.
2) The producers presumably did not help financially the laboratories. Laboratories had to make an investment to work with APS. Most didn't. That alone would have killed APS, a new format needs acceptance all over the productive chain.
3) The old same VHS - Betamax story, or the PC effect. Once there is a huge "installed base" for a certain platform, for a new platform it is quite difficult to entrench itself in the market. People want to use their investment. Professionals need to use their heavy investment. Even if the film surface had been deemed enough quality-wise, professionals would not invest in a totally new system (new camera and new lenses at least). Again, professionals acceptance is the key to the high-end amateur market, where probably most of the money is.