APS was on paper a very intelligent format.

Imagine projecting slides without having to mount frames, organize them into carriers and change them.
Imagine archiving films in their canisters with a clear record (on computer if needed) of their content.
Imagine changing rolls mid-way and reloading them without risks and without waste of film.
That, besides the solving of the loading "problem" (OK not really a problem but in any case a cartridge like APS or 126 or 110 inspires more confidence in the non-photographer).

Besides having appeared on the scene "late" and being overcome by the digital wave I think its main problem was to have chosen a format smaller than 135. That was evidently done in order to favour the production of small, pocketable cameras but it would certainly have discouraged serious photographers from adopting it.

If they had chosen an APS a little larger, with 135 frames, they would have probably had more success for the interoperability between 135 and APS (choice of traditional slide mounting or not, traditional sleeve archiving or not, easier compatibility with existing enlargers, and easier design of SLR based on existing design).

I suppose the APS format was since day one aimed at the the pocket camera market and not really meant to replace 135.