There are a lot of changes. Is "b, p, r, <click>" better than "b, d, enter, enter"? Probably not but it doesn't seem worse.
Only if you have three hands. Otherwise you need to move one between the mouse and keyboard.
Yeah, I agree. If you cut out the mouse entirely then it's very hard to argue that literally any hotkey on the keyboard is not at worst a small hand movement away. Especially when you consider that the amount of times you use it is basically compensating for how difficult it is to remember. Need to use a hotkey a hundred times? Even if it's something like shift + random hotkey you're going to pick up on that and have it memorised quickly. If it isn't an important hotkey you're not going to have to use it very often so looking it up doesn't seem to be as much of an issue (looking things up in DF is not hard).
Having to re-settle your hand to the mouse, locate the cursor and move the cursor where you want is a non-trivial thing to do (and re-settle back to the keyboard, I suppose, but I don't really count that). Certainly more than enter twice, something everyone knows how to do without even looking. It's not especially difficult to switch to the mouse like that in the sense that it's something anyone can do by shaking the mouse (especially if you've got that "shake to illuminate" windows feature enabled) but I find arguments that the new UI is anything but strictly worse than the old UI for people who took the time to learn what the UI wanted you to do somewhat unconvincing. It's not like you were learning something bad that wasn't any faster than a mouse, either. You were learning something that was somewhat opaque but had *a lot* of resources *and* once you learned it you could do things much faster than with a mouse and clicking everything.
The problem with a mouse and keyboard is you need to make a decision when you're doing computer input: you either use a mouse and keyboad and you lock out the entire right half of the keyboard away from a WASD user as a reasonable hotkey but in exchange you get access to a mouse (and all the advantages and disadvantages that entails) or you ditch the mouse and gain basically every key press on the keyboad (but you lose a mouse). DF clearly made the decision (I honestly have no idea why he did it but it wasn't a bad idea by any metric) to open up that hotkey territory at the expense of the mouse, and for the life of me I cannot actually think of any way that the practical use of the mouse wasn't fixed by dfhack. I mean, if you're going to try and compensate specifically for this lack of a mouse, dfhack seems to do everything you'd want while still emphasising keyboard controls. You don't get clicky buttons anywhere, but the design philosophy of hotkeys doesn't need the mouse for clicky buttons. The only places where I'd say it's measurably better to use a mouse is for designating things, and possibly for a hitherto undesigned military UI rework (I mean, not knowing about the steam UI at all) and DF hack does that quite neatly in my opinion.
People chose to use the mouse for things like windows and etc. because a clickable GUI really is a good idea for most things, I don't disagree. In fact, I am not really in favour of *not* having a clickable UI, I just think that there's good and reasonable justification for wanting and asking for keyboard only support, legacy style.