Odds of Hanging On
Bad image there. Getting through the first year of the groundsman phase is the most difficult part. Those who are too portly (which is a polite way of saying "plump," "stout," or just plain "fat") to become climbers may be groundsmen for quite a while, but become more proficient at their work.
Graduating to consulting arborist is a big change for either a groundsman or a climber, but is typically a welcome change. Yet, some enjoy climbing too much to give it up, and so tend to take the sort of inspection work that requires climbing.
Not many make a career out of arboriculture. Most make a career change somewhere along the way for something more lucrative and/or not so physically demanding.