I spent a week in Monterey, CA for DR training on Concurrent CP-DOS. Even briefly saw Gary Kildahl at the pizza party at the end of the week. The DR guys were impressive. They intimately knew every step level of Intel's 286 and were helping them debug it. They knew all the MS-DOS system calls like the back of their hand. They were developing a 680x0 version of the OS at the same time and were going back and forth between the two hardware architectures in testing and picking up design flaws. They were purists, but they were good!
Unfortunately, as you say, they didn't see the market potential of what they had or taylor it for the a single user.
For that time, their GEM GUI product didn't look bad either. I didn't play with it much.
A missed opportunity for them.
On the other hand, their C compiler was a piece of junk. They did not use it themselves for the OS work! The compiler they used was from a company that's long gone and I can't think of at the moment.