I'd say Sega's floundering indicated the opposite, despite a huge portion of that being own goals. I don't see any fundamental reason the market couldn't have been a duopoly.
And, to the GP point of crediting Spencer, there weren't even many synergies to exploit with a Microsoft console in the first XBox generation. It certainly didn't "integrate" with Windows in any way that made you more likely to buy it over alternative consoles.
AFAICT (as someone who doesn't spend much time console gaming now), its success was essentially built on the back of (1) access to capital, (2) savvy exclusives, (3) intelligent acquisitions, (4) avoiding missteps in hardware refreshes, and in later generations (5) strength of social platform. So, props where props are due, because 4/5 of those are skill. Especially while no doubt having to fight an internal battle against all the other Microsoft political power centers.
