Well, for my personal experience it ultimately comes down to a slower speed being an obvious form of padding to a game's length.
That doesn't jive with the argument that people are only asking for a minor speed increase. You yourself said
"It looks like things were definitely too fast before", implying you're only interested in a minor buff to the speed. With so much time being spent on turn-based combat I have to wonder how much time is really spent traveling at the maximum running speed, and how much shorter the game would truly be if that top speed was buffed "just a little". It's not like the game will suddenly takes half as long to beat.
Even if padding was the intention (and I don't believe it was), I don't think there is any significant time-saving to be had here.
.. so I don't particularly understand how walking at a normal pace is somehow poisonous to the immersion.
But the current speed
is a normal pace. Exactly how quickly do you think people garbed in armor and weapons should be striding across the landscape?
I do agree, however, that there are alternatives worth exploring before increasing the movement speed across the board for everyone.
Anyway I disagree that the walk clickable area is at a good size, but really a run toggle solves that so the area is meaningless. I'd rather the game always assume I want to reach an area where I click as quickly as possible, unless I specify that I want to be walking. The arbitrary area of clicking creating a change of movement behavior is the issue here, when it should be at the player's discretion.
I invest in Perception and the Lightfoot talent (which helps me to see traps and hidden objects). Because of that, I like to make that investment pay off, which requires not speeding about everywhere (particularly indoors). The walking radius is already small enough; any smaller and it's going to get frustrating and/or difficult to trigger.
As for the transition animations between walking and running.. it's an animation effect I really like. I actually get quite irritated with developers on this point because these blended movement animations are actually a fairly common feature in modern PC games, but unless you're using a gamepad with an analogue controller you don't have access to them. Most keyboard+mouse users are stuck with a toggle between walk and sprint, with no access to slow-walking, jogging or all the other speeds in between. I think it's nice that Larian opened this up for mouse+keyboard users for a change.
That doesn't mean I think a toggle is out of the question, only that I think it should be offered as an additional option rather than outright replacing the current control scheme.
