Agreement IBM had to make with the DoJ/etc in the 80s to open the PC platform to avoid antitrust prosecution. That was the key event.

I would argue that the key event was Columbia Data Products’s clean room implementation of the BIOS.

https://en.wikipedia.org/wiki/Columbia_Data_Products

That, and I’m pretty sure the DOJ had ended the antitrust suit (which was about bundling) by the time the PC was released.