Because Microsoft supports critical applications that businesses use for well over ten years in a lot of cases. Zero other operating systems have to provide the platform stability that Microsoft does.
So, of course, if they have to focus on supporting only one, archaic platform, they never move forward or create anything new or improved. Which doesn't make sense. So you continue to create newer, better platforms, but continue to support the old ones for years to come. Eventually, older ones fall out of use and can eventually be retired.
What would happen if the vendors got together as an organizational body (or their employees as a union/guild) and decided to just not provide the enterprise market with even one locked-down-and-supported option?
Then the one company that either bucks the trend or decides to enter the market after them with a locked down and supported option wins the market by default.
Are you imagining a union without any sort of industry-wide enforcement of disbarment for professional malfeasance? That's a pretty nonstandard interpretation of "union."
Presumably, if it's a union disbarring people for doing something, then it's something the majority of its members feel strongly about. Like how, say, civil engineers feel about buildings being built to code.
Given that the union I was talking about above is specifically one that would be formed by ISVs to protect its member employees from being coerced into bad working conditions by their managers or the clients their demands come on the part of, I assume that "not having to maintain software in perpetuity with increasing labor-load" is probably one such ideological point they'd be likely to stand behind, among others. (Or it might not be; either way, I still think "union that can disbar programmers" is an interesting solution to the problems that programmers do care most about, whatever they may be.)
Remember, the point isn't to punish the disbarred engineer--they likely were coerced into the practice. The point is to signal to the companies who would attempt such coercion, that it will result in their engineers being removed from them, so they shouldn't bother. (Yes, the company's name might also be blackballed in the industry, but that might not matter to the company if they're still able to make money. On the other hand, having no engineers who would ever want to work for you, for fear of what it would do to their careers, would matter.)
This is only partially true. A large issue is that most of the software that runs on top of their products is also locked down. So when people have business critical software that's no longer supported by the original developer there's no way to fix it if the operating system gutted support for something they depend on.
Tailor-made business-critical software is supported in a completely different way than your average shrink-wrap.
"Sure we can update from Win32 to $modern. We estimate it to take between thirteen to seventeen man-months. You know the usual rate for a man-month, let us know when you want to start the project."
The open web and open frameworks supporting it have changed significantly over the past 10 years. It's "keep the old stuff running" or "rewrite" there too.
So, of course, if they have to focus on supporting only one, archaic platform, they never move forward or create anything new or improved. Which doesn't make sense. So you continue to create newer, better platforms, but continue to support the old ones for years to come. Eventually, older ones fall out of use and can eventually be retired.