Nobody said it was by accident. They had deals with PC manufacturers for decades where they sold licenses based on the number of PCs sold by the manufacturer regardless of if the PC used MSDOS or another OS (like DRDOS), making all other options more expensive unless the client asked for them.

But the important thing is that the clients pretty much never asked for other OSes because all of their software worked on MSDOS - and later Windows. People bought computers to run their software, so if the software they wanted to run needed MSDOS or Windows, they'd buy the machines that ran that OS.

And by extension, if the software they wanted to run wouldn't run on the next version of MSDOS or Windows, they wouldn't have a reason to upgrade MSDOS or Windows. But from a user's perspective MSDOS/Windows was the best choice because everything supported it.

Microsoft didn't rely on just backwards compatibility (especially since the idea of "backwards compatibility" relies on something to be compatible with in the first place) but it was an incredibly important aspect of their strategy.