You might be right, but I'm not sure the timeline lines up.
The Magic Trackpad didn't exist when the Magic Mouse 1 (the one with AA batteries) was designed. So they definitely cared about the design of the Magic Mouse at that time.
The Magic Mouse 2 (the one with that added the bottom charging port) was released on the same day as the Magic Trackpad 2, and the Magic Keyboard 1. The three devices were almost certainly designed together, likely by the same industrial designer. And, because the Magic Keyboard was a ground-up design at the time, this would have been one of the more-senior designers doing a complete design cycle, aiming to create a coherent "peripheral brand image" to suit the marketing of a new generation of Macs.
If that designer chose to do very little to the (external) design of the Magic Mouse 1 to update it to the Magic Mouse 2, that might be because they were taking operational-logistics concerns into account, e.g. a stock of existing aluminum housings + multitouch-digitizer-laminated plastic covers. But, more likely in my opinion, they just thought that the IXD of the Magic Mouse 1 already achieved its goals (in Apple's conception, not necessarily the consumer's!), and already aligned with the brand image they wanted for the Magic Trackpad 2 and Magic Keyboard 1.
Remember that the Magic Mouse is a mouse. You need to grip it and move it, and it needs to have a certain amount of inertia so that bumping it doesn't shoot it off your table. So it needs to have a certain weight and a certain height.
I have a strong suspicion that, back when developing the MM1, Apple's design team invested into a design-prototyping human-factors-analysis phase, to find an optimal height and weight (and center of gravity!) for the Magic Mouse, so that it would "feel good in the hand" and hit some optimum between "gliding and clicking well as a mouse" and "resisting running away from you when used as a multitouch surface."
If I recall consumer reviews at the time, the MM1 was taken as a step-change in the "prioritization of function over form" of Apple's mice. Before the MM1, Apple's last ergonomically-satisfying mouse had been the Apple Desktop Mouse II back in 1992! In the late 90s/early 2000s — that's the iMac "puck mouse" and Apple Pro Mouse era — many people had been just tossing out the mouse their Macs came with, and buying PC mice instead!
Since the MM1's (very likely) evidence-based design had been so successful, the designer of the MM2 probably wanted to reuse the "backed by the research" numbers the MM1 had arrived at. As long as human hands are human hands, those will still be the right numbers (at least when viewed through the lens of Apple's internal IXD-culture biases.)
There are several ways the MM2's designer could have aimed to hit these same numbers — but the simplest way to do it (provided the old design still "fit" in the new line-up) would be to keep the external form-factor the same (thus keeping the height and grip the same), and add just enough lithium-ion capacity inside the device, in just the right place, to hit the same weight and center of gravity that the MM1 had.
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> both the Magic Trackpad and the Magic Keyboard have charging ports on their back, making it very easy to use them wired, which many people do
The Magic Keyboard and Magic Trackpad don't need to move. They're supposed to stay where they are, unless you pick them up. So they can maximize thinness and lightness (which looks "sexy", and is better for supply-side materials and shipping costs), while staying in place by just having really grippy feet (made of the most dust-collecting silicone I've ever seen on a device.)
Apple doesn't care if you leave the Magic Trackpad or Magic Keyboard plugged in all the time, because they're stationary. There's no User Experience "magic" you get by unplugging them. Unplugging them is convenient in certain limited-space environments, and de-clutters your desk, and looks good in product photos — but it doesn't make using them better.
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ETA: I looked into what actually changed between the MM1 and MM2. You might be surprised!
Keeping the same external form-factor, doesn't mean that the MM2 was just "the MM1 with a lithium cell where the batteries had been."
Here are fully-disassembled images of the MM1 and MM2, c/o iFixit:
• MM1: https://guide-images.cdn.ifixit.com/igi/TcIwDRPZ4WfdvjJF.hug...
• MM2: https://guide-images.cdn.ifixit.com/igi/ovXNR4Y3aAUYXGNj.hug...
So, for starters, there was clearly a complete internal redesign and rework of the board. Different ICs, different layout — even a different sensor, in a different package, from a different vendor, with different optics.
Also take notice of how the charge controller is integrated onto the MM2's mainboard. There are companies that have transitioned pre-manufactured devices to a rechargeable rev, by "just slapping a charging port in where the battery door had been" — and those companies tend to stick the charge controller and charge connector together to form a little floating board, and run flying leads from that floating board to the mainboard in one direction, and to the battery cells in another, so that the whole charging assembly together "presents as batteries" to the mainboard, whether it's being charged or not. This was not that kind of hackjob.
And actually, the external design changed in several subtle ways, too. Note the differently-designed runners that interface into the housing in a different way, for example. Note that even the digitizer connection to the underside of the touch surface is different — which probably implies a different digitizer, and means that they couldn't reuse the existing acrylic top housings (that they almost certainly get shipped to them with digitizers already laminated in.)
In other words: the MM2 didn't reuse anything! These are entirely different parts, that just happen to look the same on the outside.
There would have thus been no parts-reuse advantage in putting the charging port where they did. It was a "free choice" — they could have put it anywhere. They were milling out new, different aluminum bottom housings (that have more material than before, so they can't just be reworks of the previous rev's bottom housing) — there was nothing stopping the designer from putting a little hole in that bottom housing on the front side!
(Nothing, that is, other than the designer's likely belief that the MM1 form factor — where the bottom housing of the mouse tapers thinner at the front and back so that the top housing basically meets the mousing surface — was some kind of Good, Evidence-Backed Ergonomics, that they would be sacrificing if they made the whole mouse body a few mm taller to give a front-side port somewhere to extrude from. I repeat what I said before: this was an ideals-driven choice, not laziness.)