Anything that requires doing registry edits means it's not something that's accessible to the common user is my point.
Until it just works out of the box, without needing you to manually enable anything, it's still a problem, and why you should work to avoid having users download files in WebP format to begin with.
Me personally, I use Directory Opus as a drop-in replacement for Windows Explorer, which has its own photo viewer that is vastly superior to Windows', so the issue doesn't apply to me personally, I'm just thinking in regards to the average end user.
This is not at all an argument against supporting WebP as an uploadable image format, just an observation.