To be fair DF's menus have been need of a clean-up for a while. Requiring mouse usage to navigate them really wouldn't help though.
Aye. The problem isn't the lack of mouse. The solution isn't to
allow the mouse.
(Noting that mouse is useful
and already usable in 'painting' designations on a map, though I personally find keyboard more accurate in this, excel and sometimes even in things like GIMP where there's key-control for precise dragging of overlay regions. I might suggest that 'rectangle stretching, such as for bridge-footprint sizing, might be a candidate for corner/edge/centre dragging for resizes/reposition, but given the expanse limitations already in bridge/road/etc designations and the arguably greater need to reconcile something like a copy/paste of such things with the possible adjustments to the material picker selections at each additional stage, perhaps we should be asking for something more like 1-click flood-fill (well, 2plus-click, right click to menu, select flood style/boundary conditions and then try to fulfill materials?) floor-building. Some people may prefer room-from-furniture size to be mouse-controlled, but as that's essentially a 1d 'radius' thing then it'd be best tasked to mouse-wheel than drag, if by usurping mouse-wheel-zoom you don't cause further UI headacjes and inconsistencies.)
We could argue over the menu 'clean-up', and what needs to be redone where. Given we can't
remove the Construction sub-items, or Workshop Autolabour on/offs, from the whole tree of submenudom. Free of necessity to tie to a (sometimes counterintuitive but necessary-for-local-uniqueness) key-combo then perhaps there could be some shuffling together of things that are more alike than suggested at the moment, and do more "boxes and bags" note of all things functionally the same (coffin/sarcophagus/casket, etc).
The point about the "three types of cursor key" (or is it four, technically?) is the other argument for needing rationalising, though there is more rationality between them than some people appreciate, and times when multiple cursor-controls are needed/useful in an operation, simultaneously, and though cursors+mouse might be one solution I'd be annoyed to find I
had to reach for the mouse because I can't do it all on keyboard like I always did.
Way beyond my pay-grade to answer the question of "what specifically needs changing", as well as probably beyond my ability to distance myself from the years of ingrained acceptance and envelopment of all the little quirks that are now second-nature to me, but if you'll excuse the above ramble, I know it's not going to be easy without an overhaul that is problematic in itself. As has been discussed at length (and width, and height, and extent) elsewhere already, though doubtless it will again, and now on the Steam Communities!
(Or Itch ones? Couldn't find any similarly anarchic Itch fora, but I must admit that I first realised that Itch was
not the same as Twitch only last night. Not having frequented either of them until I went looking for what the new audiences would find.)