Multiple rockets, such as the Atlas V or Saturn V, have used the combination of a kerosene/liquid oxygen (kerolox) lower stage for higher density and thrust within the atmosphere and a liquid hydrogen/oxygen (hydrolox) upper stage for better efficiency or specific impulse in a vacuum.
SpaceX has decided to avoid the use of hydrogen on their Falcon 9 rocket, which is said to be an "optimization for cost," as liquid hydrogen is very cold and therefore more expensive to handle. The Falcon 9 uses the same kerolox fuel and mostly the same engine on both stages to take advantage of commonality and mass production. The lower specific impulse however gives it a disadvantage if you are sending a probe on an interplanetary trajectory, but the reduction in systems, manufacturing, and operating costs is said to be worth it for most launches.
SpaceX's Starship rocket currently in development plans to use methane on all stages as it can be stored for long periods of time (liquid hydrogen can boils off and leak out), can be made on Mars more easily, and is good for reusable engines as it produces less carbon deposits (coking) compared to kerolox and less hydrogen embrittlement compared to hydrolox.
However, most other launch service providers that have already developed hydrolox engines are still planning to use them, such as Blue Origin. If another company or space agency has already developed hydrolox engines, would a switch from hydrolox to methalox be a step backward since a new engine (which has less specific impulse compared to hydrolox) needs to be developed, or a step forward in terms of reducing overall cost in the long-term?