Yer Hired.A gearbox is something that usually does need replacing/overhaul in due time. How long they last is a function of the maintenance protocol and service load. It's a piece of equipment--in all industries--that is usually towards the top of the list for a predictive maintenance program...oil samples are usually taken and analyzed for signs of mechanical wear---often in the form of metal shavings/particles found in the oil, water content of the oil, viscosity of the oil, etc. It's all about trending---samples should be taken and analyzed periodically to look for how those metrics are trending because that's what indicates a looming problem.
Listening for unusual sounds before shit breaks helps too.