They will last virtually forever. Hardened steel gears are what’s used for nearly all automotive gearboxes. My gears are 4140 alloy steel with a nitriding heat treatment. I actually opted to leave the wheel gear unhardened to save some cost since its largely unnecessary. All gears wear but the wear is very very minimal. The bearings would be the first thing to go.
The cost of gears this size/precision hardly depends on material. I can get POM gears made for basically the same price but steel is cool