And Kindle supports mobi files? It is just that those tend to get preprocessed into azw or the other one files. Much like Kobo tends to work best if you preprocess those epubs into kepubs.

The issue is that Amazon has repeatedly changed their mobi variants to fight against de-drm tools as well as increasingly locking down their apps and even devices to make it harder to get data off (and now on) to them.

There is absolutely nothing stopping Rakuten from doing the exact same with Kobo. And people should be aware of that rather than just stanning their favorite company.