That it would be different in inarguable. I feel like I can get on board with the statement that the fact they are no longer the biggest is pretty much equivalent to saying they lost their monopoloy.
In a sense they have a monopoly -- they are the only ones selling Windows. But ... even in the desktop/laptop area they are no longer the beast that they once were. And the prevalence of mobile as a platform means that even their dominance in the desktop/laptop market doesn't mean what it once did.
In office products they have a much stronger position, but the barriers to switching are no longer what they once were. I feel like I can confidently say that Microsoft can no longer coast on their monopoly power, but must actively move forward (with things like Office365) in order to stay competitive.
Are you arguing that even now breaking up OS/Office would be a reasonable thing for the Justice department to pursue? That seems like a barely tenable position.