It doesn't matter if the UI is better fit for phones. It doesn't matter if it was the best UI design in the world. The new UI sucks because it isn't Discord. Many of us who use discord mainly use it on PC, and only use mobile for small casual things or call if we really want. We're constantly switching between the two devices, so the design of both platforms should be similar. You could swap between the two with little to no problems when navigating the app. And now Discord just got rid of that benefit. Now the mobile design rearranged a bunch of things, menus coming out of different directions, and buttons that don't exist. Not only is it a new thing you gotta learn how to use, but it's more disorienting to switch between the two devices now that they pretty much have different UI designs.
