Webp may be great for displaying in a web page, but it's TFU if I save it to my hard drive and try to view it with default viewing or editing software.
It may not be a proprietary format, but it might as well be.
I'll admit that support for the format isn't perfect, but at least in your case it can be remedied pretty easily with a quick search [1]. That said, format adoption has to start somewhere. It'd be pretty sad if we were still using GIFs for all of our lossless images because PNGs were never allowed to catch on before they could become universally supported.
Is webp added in qt5?
BTW: gthumb opens it A-OK. So, maybe the issue isn't the openness of the format, but the openness of your platform?
Webp is in a lot of the same boat as webm legally. It may be "open", whatever the fuck that means anymore, but that doesn't make it patent free or not needing of any licensing concerns. VP8/9/etc... aren't some public domain web format. Even webm infringed on H264 patents as of 2013. Do you really think that is an encouraging thing for lawyers of companies with legal liability to want to risk?
Where's your proof?
I'm surprised preview doesn't do WebP, that's too bad.