This is indeed frustrating, though realistically not that big of a deal for most users, especially if you consider lossless as just highest quality. Though personally I think it makes sense to have different extensions by convention for lossless and animated images.

It's a big deal :(

JPEG artifacts are literally a meme, normal people are aware of this stuff.

I hold the extremist view that had we decided to use .png for lossless webps, it would have been an overall net gain - in people's minds "png" maps roughly to "whoever last touched this image didn't do anything evil to it" - very few people care how the data is actually encoded.