At the time, IBM made very expensive mainframes and minicomputers, plus all the associated paraphernalia: terminals, printers, storage.

It didn't make microcomputers. It didn't really sell to the general public. It sold smaller kit to big businesses, and the bigger kit even they couldn't buy: you could only lease it.

Microcomputers were only just beginning to be a product sold to the public for home use. Up until the first few complete, integrated systems in around 1976-1977 (in the USA, the Apple ][, Commodore PET, and original TRS-80), microcomputers had been expensive modular devices, used in business.

That's the market DR sold to, and utterly dominated: relatively price S100-bus small business micros.

DR was not _a_ big player in microcomputer OSes, it was _the_ big player. Almost the only player: I can't think of any other cross-platform microcomputer OS available in the 1970s.

The PC was a highly atypical product for IBM, made from COTS parts, intended to be sold to the public, direct to end-users. It's not really something anyone expected IBM to do, and in this position, it was a company nervously trying to get into a totally new market for them, approaching the industry giant with caps in hands.