It depends. Personally, I keep my animals for life. But I am fortunate to have the space and therefore the means to do so. And my horses live out 24/7. Not everyone has the resources though. The high costs involved in keeping a horse can make it hard to hold on to them forever, even if you are reluctant to sell. Its not their sale price as such but the cost of keeping them fed, housed, with regular teeth and hoof care, vets bills, tack, transport, lessons, etc. Its all very £££.
In the case of schoolmaster ponies who teach dc to ride, you will have the well trained ones with good temperaments passed around horsey families, but "passed around" in this instance means highly prized, very well cared for and loved, and living with knowledgeable people for five to eight years or so until the DC become too tall/heavy and move on to horses. So it will only move perhaps twice or three times in a lifetime and only to good homes. They rarely come on to the open market and are usually bought via word of mouth. You often find an "old" pony like this living a happy retirement with its last family.
In this instance I think a sale is justified because a good pony has a lot of miles and a wealth of experience and training behind it, and obviously small children have to learn on something very safe. And once the child or children are too big for it, it would be a waste of its experience and training to do nothing if it is still young enough and fit to be ridden by another child.
And despite what a lot of people think, a horse living with one or two companions in a featureless field doing nothing for years on end , isn't always the best scenario. Some horses are chilled and are fine with it. Some are miserable and need more stimulation, given that horses in the wild have to work a bit to find food and water and are constantly on the move, encountering different landscapes etc. My current horse is happy with his mates in the field but he absolutely loves to go out in the trailer to a show or on a ride in a different area for example and he is generally very curious about people and likes lots of attention from them.