If they are understaff (namely giga honest, super solid competent people for air traffic ground control), you have to reduce air traffic as it is way too dangerous. For this kind of staff, it is not just a matter of "training", it is way more demanding.
They won't cut traffic for the same reason they are understaffed: money. Raise pay until you have enough staff. It's not always that simple, but sometimes it is. Not everyone could do it, but a lot of people could. You just have to pay them to attract them from more lucrative jobs (which includes training them to do it). Instead they'll pay as little as they can, and occasionally pay to replace an aircraft.
Pay isn't enough, you also have to give the plausible impression that their job will still exist in a few years, and they won't be laid off on the whim of a shadow president.