[rkagerer]

This is truly unhinged

Granted, but at the same time it's also resolutely pragmatic.

Apparently there was already lots of software out there which expected to be able to write new versions of system components. As well as buggy software that incidentally expected to be able to write old versions, because its developers ignored Microsoft's published best practices (not to mention common sense) and and didn't bother to do a version comparison first.

The choice was to break the old software, or let it think it succeeded then clean up after the mess it made. I'd bet they considered other alternatives (e.g. sandbox each piece of software with its own set of system libraries, or intercept and override DLL calls thus ignoring written files altogether) but those introduce more complexity and redirection with arguably little benefit. (I do wonder if the cleanup still happens if something like an unexpected reboot or power loss happens at exactly the wrong time).

Could the OS have been architected in a more robust fashion from the get-go? Of course.

Could they have simply forbidden software from downgrading system components? Sure, but it'd break installers and degrade the user experience.

Since the OS historically tolerated the broken behavior, they were kind of stuck continuing to tolerate it. One thing I learned leading groups of people is if you make a rule but don't enforce it, then it isn't much of a rule (at least not one you can rely on).

I would argue the deeper mistake was not providing more suitable tooling for developers to ensure the presence of compatible versions of shared libraries. This requires a bit of game theory up front; you want to always make the incorrect path frictiony and the correct one seamless.

[badsectoracula]

There was (and still is) VerInstallFile, however this was introduced in Windows 3.1 and it is possible installers wanted to also support Windows 3.0 (since there wasn't much of a time gap between the two many programs tried to support both) so they didn't use it.

[xenadu02]

It is important to remember that Microsoft created some of this chaos to begin with. Other aspects can be attributed to "the industry didn't understand the value of $x or the right way to do $y at the time". And some of this is "nonsense you deal with when the internet and automatic updates is not yet a thing".

Why did programs overwrite system components? Because Microsoft regularly pushed updates with VC++ or Visual Studio and if you built your program with Microsoft's tools you often had to distribute the updated components for your program to work - especially the Visual C runtime and the Common Controls. This even started in the Win3.11 days when you had to update common controls to get the fancy new "3d" look. And sometimes a newer update broke older programs so installers would try to force the "correct" version to be installed... but there's no better option here. Don't do that and the program the user just installed is busted. Do it and you break something else. There was no auto-update or internet access so you had to make a guess at what the best option was and hope. Mix in general lack of knowledge, no forums or Stack Overflow to ask for help, and general incompetence and you end up with a lot of badly made installers doing absolute nonsense.

Why force everyone to share everything? Early on primarily for disk space and memory reasons. Early PCs could barely run a GUI so few hundred kilobytes to let programs have their own copy of common controls was a non-starter. There was no such thing as "just wait for everyone to upgrade" or "wait for WindowsUpdate to roll this feature out to everyone". By the early 2000s the biggest reason was because we hadn't realized that sharing is great in theory but often terrible in practice and a system to manage who gets what version of each library is critical. And we also later had the disk space and RAM to allow it.

But the biggest issue was probably Microsoft's refusal to provide a system installer. Later I assume antitrust concerns prevented them from doing more in this area. Installers did whatever because there were a bunch of little companies making installers and every developer just picked one and built all their packages with it. Often not updating their installer for years (possibly because it cost a lot of money).

Note: When I say "we" here that's doing a lot of heavy lifting. I think the Unix world understood the need for package managers and control of library versions earlier but even then the list of problems and the solutions to them in these areas varied a lot. Dependency management was far from a solved problem.