The "support elements" on the mainframe, which are just a laptop connected to some internal buses and power controls, did run OS/2 for many years. You could IPL the machine right from the Warp desktop. It was neat.

They also continued VM with VM/370, VM/ESA and z/VM operating systems which are equally rock solid systems. Aside from these IBM did actually go big on Intel systems but they got completely blindsided by clone manufacturers and entirely failed to see what cloud infrastructure would do to their impressive mainframe lineups.