Designers are always looking to make lighter, more aerodynamic planes, with more efficient engines. But even if radically more efficient planes were devised, it takes around 15 years from design to operation.
Efficiency increased very quickly in the 1960s, but has now slowed and is around 1% per year. This is outweighed by the bigger increase in passenger numbers – 5% a year between 2010-2019 (www.statista.com/statistics/564717/airline-industry-passenger-traffic-globally/), and Airbus still projects 4.3% growth per year, so emissions continue to rise. This is illustrated in this chart.
Also, any increase in efficiency will result in cheaper flights, which will increase demand and mean the efficiency improvements are used to fly further or more frequently, rather than reduce fuel use, unless emissions pricing are introduced.