External to the CPU - of course.There are nontrivial efficiency costs to moving RAM and storage external.
External to the display - no. If parts replacement is your priority then a Mac Mini/Studio + separate display is your answer. The Mini/Studio SSDs are already on replaceable modules (it's upgrading that Apple are blocking) and even though the only cure for RAM failure is a new logic board that can be done without taking a pizza cutter to the display and having to replace all the adhesive strips.
Yet the mini and previous iMacs have always got on fine with internal PSUs - since when processors have only got more power efficient. Apple have even managed to make the Mini ridiculously small and keep the internal PSU. The big thermal limitation with the iMac is having the CPU/GPU/Storage and display sharing the same unnecessarily small housing.The thermal benefits from moving the power supply external would also be nontrivial for small, thermally restricted designs like the iMac and mini.
The PSU was pretty obviously moved from the Mini purely for the sake of making the display an ultra-thin slab for no practical reason. With the handy upshot of making it too thin to accommodate an ethernet socket... Previous iMacs used a clever tapered design to accommodate what was needed while still looking slim.