Bigger battery = The main weight of a DSLR body is already the battery. They can't go the Apple route of filling every possible space in the case with battery e

Bigger battery = The main weight of a DSLR body is already the battery. They can't go the Apple route of filling every possible space in the case with battery either because it needs to be replacacle. If the reason for getting rid of the mirror is mechanical reliability I don't think it's a good idea to add something else that can break. There isn't a technical reason, but rather which trade offs do you want to make? You can make a mirrorless camera that has every feature of an DSLR, but there is no point to do that if you lose the advantages of a mirrorless system (small and light).