> I’m aware of alternate mainframe OSs but I’m not sure how common using one was.

Extremely common at major universities and research centres. CTSS, ITS, TENEX, Multics, Unix and even VM/370 were all alternate operating at some point.

> Other than OS2, alternate OSs for other systems were rather rare,

You weren't there, were you? A lot of people replaced MS-DOS with DR-DOS before Microsoft deliberately broke it with Windows. A little later, a number of people were running Unix System V on their PCs, to the extent that there was a regular column about Unix in Byte.