It's not about the DM button being moved, it's about all the UX they broke in halves. They changed how swipe works, they changed how the DM list works, they changed how search works, none in a good way.

For a lot of "power users", changing how swiping acts goes in direct opposition to the muscle memory, so that's annoying. Some functionality is just gone or is made much worse, like no longer seeing images in search and no longer having autocomplete for the filter terms there. You can no longer look at the DM list and then go back to active DM because swiping there was turned into a dumb back button replacement.

And above all, this is just a canary in the coal mine moment - they do not care about their current app, they will do whatever they like and people are understandably uncomfortable with that.