I think the explanation is simpler than that. To Apple, the MM1 design had already reached some Jobsian perfection and didn't need to be redesigned whereas the keyboard and trackpad were able to be simplified and made more slab-like when the batteries were removed.
Everything I hear about Apple is that they are (or at least were) a design-first company. It wasn't enough that it was only a one button mouse, Jobs wanted a no-button mouse. Look at how much effort went into hiding the functionality of the mouse-- that one surface is not only two buttons, but also two directional scroll and various other desktop gestures. An entire multitouch surface was designed and integrated into a mouse to hide a button at a time when multitouch was still pretty novel even in phones.
Now they have it, and someone wants to disrupt the smooth curve of the touch surface leading down to the desktop? People might notice that there's actually a button under there-- they might see something move. They won't give up that clean design to add a charge port on the nose.
My guess is that the only redesigns coming to the mouse will be force touch and the taptic engine, like the track pad, and probably a shift to MagSafe charging someday. Less lines and grooves, less apparent function. Magsafe doesn't help with functionality while charging, but I'm sure they'd love if it didn't look like a dead beetle-- just like when they moved the Apple pencil from lightning to wireless charging. That was a lot of work to avoid the perfectly functional weirdness of a pencil sticking out the bottom of an iPad.
..At least until the remnants of the current design team move on and someone else comes in with a new design ethos. As annoying as some of these Apple design decisions can be though, the attention to design it's what makes Apple Apple, and I don't much look forward to a shift toward fully utilitarian design.
