I still have my old Magic Trackpad. I was excited for the Magic Trackpad 2. I never went back; I still prefer the look, feel, and performance of the Trackpad 2. For the environment I would prefer user-replaceable batteries but my current Magic Trackpad 2 is about seven years old and I probably charge it about once a month. For my desk setup, it honestly would not be a hassle to keep it plugged in all of the time; wireless mice were a big deal, trackpads don't really need to be wireless for most of us.
Why not make the thing a little bigger (to fit a larger battery), and also more ergonomic at the same time?
We'll never know for sure, but most people blame Jony Ive, who was Apple's lead product designer. It's generally felt that he had an obsession with thinness. I share your views and I would scratch my head when a new product came out and they said that the battery life was the same, but now it was even thinner. It seems like they could have kept the same thickness (which was already class-leading thin) and increased battery life, right?
Jony Ive is now formally out of Apple, leading his own company that provides guidance to Apple but probably isn't directing much anymore. We'll never really know how far his designs went - I'd like to think that the iPads becoming a bit chunkier now and not having tapered edges represents leaving his designs, as well as the Mac Studio being a chunkier Mac mini (the system could still have afforded to be larger, but the fact that Apple didn't try to make something even smaller is still a good sign), or the Mac Pro leaving the "trash can" design and becoming larger and more practical again... who knows? Maybe the next iteration of their keyboards and pointer peripherals will be a bit larger and more practical, too. I'm hopeful.