Microsoft did not grow big, because millions of home users decided to buy DOS/Windows as they considered the best choice. Microsoft purchased DOS, which was bundled for every single IBM PC. This gave the foundation for the company to be able to improve their software family giving them a stable income even if every single DOS alternative was superior to their system. IBM was not able to prevent the creation of the PC compatibles which was a really strong advantage for MS as DOS became their default go to OS. Cheap computers created a really big market for the platform so developing to MS products became a good idea. However the businesses were the ones who choose to buy Microsoft products, and not because of the OS, but their office software products, which worked, of course, primarily in their OS. Most of the home users just get it bundled with their hardware purchase, others for home use just pirated it (especially out of US). Their focus were the software they use, did not really care about the OS. And that's how piracy helped MS, children playing with computer games arrived in the work market with familiarity with MS products, which kept the companies using and buying those products.