Just a nit, but OS/2's original problem was that IBM insisted on it being able to run on 286's. Mainframes were not a special issue for OS/2, as even Windows had to play nicely with them, and always has, with decent 3270 and 5250 emulation software.
Subsequent problems for OS/2 relate to the fact that IBM had a bunch of dunces in charge at every step of the way.
Anyone curious about how bad a job IBM did with OS/2 should read Gordon Letwin's famous usenet posting on the topic:
http://groups.google.com/group/comp.os.ms-windows.misc/msg/d710490b745d5e5e?&hl=en
“Mainframes were not a special issue for OS/2, “
Not true, some technical issues but many more politically. I have had a lot of direct experience with IBM field reps poo-poo-ing OS/2. We needed a small database server at a large bank and the entire IBM team and some sent in from HQ dissuaded us. We got a Sun. Ran ORCL which actually had much better DB2/DB2 integration!!! Worked great. This was ORCL 6.32 the most solid ORCL ever.
This was part of IBM’s attempt to moat CICS and kill C/S computing.