Modern washing machines already have a bunch of sensors, some including sensors to weigh the load and act accordingly.

And you keep talking about retooling factories. Manufacturers don't continually turn out the same* appliances year after year, they update and redesign them all the time. Adding an extra slot that is connected to the firmware and electronics they already have doesn't involve retooling the factory anymore that updating to next years models does.

* or at least they don't in Europe, though given the pictures I've seen of US appliances in the past, the same might not apply there