Actually, it was the other way around. They became a monopolist because they managed to dominate the market. And they managed to pull that one off against all odds: a small garage company against a giant like Digital Research, but they were lucky (or clever) enough to sign a contract with IBM who were just introducing their new product line called PC. IBM did not expect the PC to compete with their main business model, the mainframe, and did not pay too much attention to it. To such an extent that they did not patent the technology and left it open for others to clone. We all know the result.

So we have come the full circle again. Open does win in the end. Even though Microsoft's software is closed, it ultimately owes its success to riding the wave of open hardware.