With proper support, the IIGS might have been, but Apple went out of its way to kill it. Go figure.
They produced and sold it for six years, which is a relatively long time for a computer model.
It probably didn't help much that the Apple IIGS got them sued (again) by Apple Corps. Apple's last update to the Apple IIGS was 1989, the same year of the lawsuit. After the lawsuit, they didn't update the Apple IIGS anymore and they didn't add the sound system features from the IIGS into the Macintosh line.
The lawsuit could be relevant, but my recollection is that Jobs killed it because it was too much possible competition for the Mac line. (I used to read a lot of computer magazine.) If I recall correctly, there was a 11GS on the drawing board that would have been faster than the existing Macs. There were also companies expressing interest in buying the II line from Apple--which wouldn't have made much sense if there were ongoing litigation concerns. I also remember Apple misrepresenting its own sales figures (by omitting all the sales to schools, among other things) to justify stopping development of IIGS. Again, had there been ongoing concerns about litigation, Apple could have just said that instead of making up bogus sales figures to support what it was doing.
I read a little bit about the lawsuit. It sounds as if that would only have affected the multimedia capabilities of the IIGS--not in itself a reason for dropping the line, since, as you point out they didn't add the capability to the Mac, either. In any case, the trademark wrangling continued until 2007, so if the goal of ending the IIGS was to prevent more lawsuits, it wasn't very successful.
To go back to the original point I was making, Microsoft isn't the only tech company that sometimes doesn't deliver what its customers want. The Apple II users at the time were numerous, militant, and wanted the line continued in some form. Apple said no. All of the Apple II users I knew--admittedly not a very big data set--are now PC users.
But if you don't like the Apple example, we could also talk about flops like laserdiscs--supposed to be a revolution in education and home entertainment. Later, of course, there were some formats that made it and a larger number that didn't. (Video CD, Digital VHS, Audio DVD, HDDVD).
I sometimes wish indie authors had the resources to do more extensive market research. Then I realize that even companies with that capability don't always do well in the market.
