I have to say that I find the statement that owning more than one is abusive to training a bit offensive.
Owning more than one is not abusing the training of one to ride another. I have 11 myself. Aside from the 2 that aren't old enough to ride and my mare that is 9 months in foal, they all get worked on a regular basis. My seniors only get me once a week and lesson students for 4-6 hours a week, everyone else gets worked 6 days a week. My youngsters get groundwork 6 days a week, my broodie I just stopped riding a few weeks ago because she is getting big and grumpy. I spend several hours a day riding just my own, plus several hours a day riding for others. I don't think in any way that is abusing their training. I love them all and they each get their fair share of my time.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.