It is silly to have both, and transponder is inefficient in the sense of using a lot of electrical power unnecessarily. The move away from Mode S to UAT makes this a lot more likely.
Can you elaborate a bit, Silvaire1?
1090ES, the datalink by which a Mode S transponder is also capable of broadcasting its position and other information (ADS-B), operates in the same vicinity (1090 MHZ) as UAT (978 MHz). To send the same data over the same range at the same rate requires similar power. And what's more, ADS-B is broadcast, so you send the data whether it's needed or not. Why do you describe the transponder as "inefficient".