The reasons might be fine, but my opinion is that is whole thing is a little bit… Blown out of proportion.
Just on a couple of your points:
The muscle memory point is very valid, but that boils down to not liking change for changes sake. Muscle memory can always be rewired, and it will take less time than you think it will.
The discrepancy between desktop and mobile has always been there, imo. So many of the desktop features were crammed away into menus and submenus that were hard to use on mobile. With a new mobile-centric design, we may not know where everything is yet, but it’ll likely be easier to access once we get more familiar with it.
The other thing for me is that Discord is the messaging app I prefer, by a long shot. If they see this as an opportunity to add SMS linking to their feature set, and allow users to then also bridge those from the mobile to desktop version, it would solve a lot of communication issues I have, having everything in one place.