Thank you for all the arguments in favor of WebP. I personally would like to point out:
Google PageSpeed Tools recommend Webp because it speeds up loading time. In my job, I can't close my eyes to this. Due to the fact that Afinity doesn't support it, I have breaks in my workflow because I have to use 3rd party tools. I didn't think I would have to deal with Gimp again. Annoying.
For me privately, I also find it nicer when my image archives are considerably none with subjectively the same quality to almost 100% JPEG exports.
The trend towards WebP became clear years ago. There are even smartphone foto apps that save in WebP.
Dear Affinity Team. Affinity Photo is innovative in so many ways. What are you waiting for?