After owning a Texas Instruments TI99-4A my 1st pc was an Amstrad Schneider PC 1512 with one floppy and one 20 Mb hdd. It ran, I believe GEM window system on top of MS-DOS.
And yes, they even came with a mouse and a GUI (GEM by Digital Research).
Sugar used the technique in products from hifi's to computers as an innovation to save money and keep costs down. That's not necessarily meant in a bad way. Amastrad often used the savings to to bump up the spec somewhere consumers cared more e.g. memory or a bundled monitor or printer.
I think when commodity PCs became wide spread Amstrad ran out of corners to cut to get an edge and became uncompetitive.
It was when Amstrad dropped their own OS in favour of DOS that they became just another commodity PC maker. Actually first Amstrad moved to DR-DOS then MS-DOS to avoid potential compatability issues. Which I'm sure Microsoft would have eventually discovered.
http://edge-op.org/iowa/www.iowaconsumercase.org/011207/PX_9...
So having their own OS wasn't the deciding factor. I think it's the more commodity nature of PC hardware even compared to CP/M machines that did for them IMHO.
And yes, by the time 286s and 386s came to the market there was really nothing more to cut, or even to add. Every part was standardized by then, profit margins shrinked and MS-DOS became ubiquitous.