I don't want to just quibble for the fun of it but so much more of this is wrong than right that it's downright misleading.
> I'm pretty sure CP/M 86 was already a thing.
Yes it was. Launched after the IBM PC, though, which is why SCP wrote QDOS which they licensed to MS and became MS-DOS.
How and why it was late:
https://nemanjatrifunovic.substack.com/p/the-late-arrival-of...
> I believe there was even Concurrent CP/M 86
There was. And then CCPM-286 and CCPM-386.
> which let you run up to 4 (or more?) virtual sessions concurrently
On the console -- but the selling point was that you could also attach dumb terminals to the serial ports and they could run DOS apps too. So you had a cheap multiuser box, where adding 3 or 4 or 8 more users cost about 10% as much as a network with 3 or 4 or 8 more client PCs.
> Light years ahead of MS-DOS but nobody wanted it,
No, not at all. One descendant is still supported today, some 45 years later.
https://tgcs04.toshibacommerce.com/cs/groups/internet/docume...
> so DRI dumbed it down and added MS-DOS compatibility
No, not even slightly.
> and re-released as DR-DOS (still superior to MS-DOS though).
Not a fair description at all.
Here's how it went:
MS-DOS beat it to market.
So, DR brought multitasking and multiuser from MP/M to turn CP/M-86 into Concurrent CP/M: CCP/M-86
MS-DOS did great. So, lots of apps.
So, DR added DOS compatibility to CCP/M.
It also did a single-user OS, called DR DOS Plus. Oversimplification: this was a single-user non-multitasking CP/M-86 that read/wrote FAT and could run DOS apps.
Intel did a new chip, the 80286.
DR ported to it. A native protect-mode 286 OS, which used new 286 features to pre-emptively multitask DOS apps.
Intel removed those features before it launched the 286. This killed DR's new OS.
So, it refocused: it temporarily dropped DOS compatibility, and sold it as an RTOS instead, with a multitasking version of GEM as its GUI.
Intel put the features back and Concurrent DOS 286 became a product but it was too late. This was a better OS/2 before OS/2 1.0 launched.
Intel put DOS multitasking into hardware in the 386. DR rewrote CDOS286 in C creating CDOS386.
This had really good DOS compatibility.
So DR updated DOS Plus to make DR-DOS: a superset of MS-DOS 3.3, with FAT16 for big hard disk, and it could run DOS device drivers.
This did OK.
IBM did DOS 4. Bigger, but with big disk support, and a GUI app launcher/file manager.
DR leapfrogged with DR-DOS 5: all that, but smaller, with built-in memory management.
A brief arms race ended with Win95 which bundled MS-DOS and so killed the DOS market.
Meantime, DR took the better DOS compatibility and made Multiuser DOS, which did well and throve for decades.
Here's a history I did 4Y ago:
https://www.theregister.com/software/2022/08/04/the-many-der...