Cherie said it quite well. There are many valid reasons for moving a horse on.
I'm another that has many "lifers", I have 5 currently well into their 20s that will live out their last days with me provided something major doesn't change my life situation. I also recently (2 months and 8 days ago to be exact) laid to rest my 'old man' and best friend since childhood at 31 years old, it absolutely gutted me and it still pains me to walk out to the barn & pastures and him not be there quietly greeting me every morning.
Then there are those that I, my daughter or hubby ride now as well as my lesson horses that spend their working time sharing their knowledge and passing on to others just getting into horses what a good horse should be.
Then there are my youngsters & personal prospects. They will be my next show horses and with some mileage, my daughters after I have them for a few years. Should they not turn out to be capable of competition at the level I have intended, absolutely I will sell them on. Should that happen they would go to homes for someone looking to show successfully as an amateur or local/open/4-H type rider, working ranch horse or a happy trail riding home.
There is not a thing wrong with moving a horse on for any number of reasons when it is done with their best interest in mind and to a proper home for that horse.
You mentioned not talking numbers but that is a huge part of it. Horses are not dogs or cats and cost inherently more to keep and maintain. I won't break it down for you but I could purchase a very nice brand new car off the lot and pay cash every year with what I spend on horses. One expense, I will be putting about 1500 bales of hay up in the next few weeks, at current prices to buy outright I would be looking upwards of 10k just for part of their winter meals. Should I get seriously injured and not be able to ride myself for a long period of time, I have backup to cover a year of me not working but beyond that, that loss of income would force me to cut back even if I didn't want to. My hubby's income alone couldn't take care of the farm expenses, family expenses and my horses. Something would have to give and the logical, sane solution would be less horses. Can't hardly sell my daughter...though some days it may be tempting
I hope that hearing responses from horse owners has shed a bit of light on the situation(s) at question and help you feel more inclined, along with your wife, to continue to do your volunteer work with those horses who need the help and try to be more open minded and understanding to the circumstances that may or may not be the reason they are there.