I don't find the registry itself to be all that poor of a concept, as someone else mentioned the issue is allowing everything to store and modify stuff there. The registry ought to be for the operating system to keep track of internal things, nothing else should be messing with it and the user should not have to edit it themselves.
Except that that everything under the sun writes to the Registry, to the point that users DO have to edit it.
I haven't had too many issues with file associations, Windows let's you right-click and "open with" in most cases and will remember the choice if you let it. I still remember what a hassle it was on the pre-OSX Macs the way file extensions were not used and if you wanted to change the type of the file you had to use a tool like ResEdit. The separate resource and data forks made it a hassle to transfer data between platforms.
Windows only remembers that choice
globally -- you can't override the preferred application for a
single file. HUGE difference.
As for Classic Mac OS, file extensions
were supported from around the early 1990s, when System 7 included the PC Exchange control panel. But they were clearly subordinate to the Creator and Type codes that indeed needed a tool to edit -- but the overwhelming majority of Mac users never even
heard of those codes, never mind needed to edit them. (Apple has since effectively recreated those codes, in an even better way, using extended file attributes, which are now widely supported across various filesystems.)
As for the data and resource forks (which are only tangentially related to the question of file associations, since the creator/type codes weren't stored in the resource fork): the fact that other systems didn't support them didn't make them a bad idea. If anything, the fact that other OSes/filesystems added and even extended the idea (e.g. NFTS's support for multiple file streams, not just data and resource) confirms that the idea was very sound. Apple's current re-implementation of this idea, in the form of bundles/packages, is brilliant, and remains IMHO one of the Mac's strongest implementation advantages, in terms of providing a user-friendly and technically robust solution to the common problem of needing something in between a dumb folder full of files, and a giant monolithic file. I am not aware of any other OS that has anything like the Mac's bundles, and they suffer for it.