Problem as I see it for broader adoption than web pages is that webP is, or seems to be, not really a picture format at all. It is a compression format like zip or 7z. The input is a 'picture' in jpg, png, or other limited number of accepted formats. Output is back to jpg, png or other limited output types, which is then supported for display. If I take a jpg and use lossless webP it came out larger, I could 7z it for lossless compression and it it smaller....
Example file sizes in the attachment:
First file is the base jpg
The second file is the list was 70% webP compression, but I left off the _70.
Of course I'm not considering speed which is a big factor with web pages. I'd bet webP is faster....maybe?
John Z