You mean the OS they failed to find a replacement for and almost went bankrupt, had it not been for the NeXT reverse acquisition?

Yes, that OS. There is more to an OS than technology. Mac OS X was the technological solution they needed, but it has ALWAYS been an inferior UI. Many hundreds of thousands of words have been written on this.

If you're at all curious, you can start here [1] with John Siracusa's famous piece on the Finder.

[1] https://arstechnica.com/gadgets/2003/04/finder/