And I’ll further suggest the (seeming) lack of older show horses is because no one is buying teenaged horses… heck, they think 14 is too old for a 1.10m horse 
But no doubt the 12-month long season, 12-week long circuits, and vanishing acreage all play a part in long-term sport horse soundness. I even question modern footing over sand, but sand doesn’t drain well and the show must go on.
I have the pleasure to know a great group of senior horses packing their 3rd-5th generation of young riders around right now (~ 2’6") and there is considerable thought, maintenance and expense undertaken to keep those horses comfortable. Most of it is shoeing… do you consider that a band-aid? There are certainly injections and bodywork— thoughtfully as each horse needs it. No one’s lining them up and throwing syringes at them like a dartboard hoping for a bullseye. And then, of course, there are the supplements and yeah, medication.
So at what point do you draw the line and toss the horse out as useless? They come out creaky and get a lot of walking, then loosen up and do an outstanding job for their kids. Kids who only have the pleasure of riding them because they come at a discount— the “maintenance free” version is 10x the money. If they suddenly start stopping, we listen. But otherwise they are, by all appearances, happy as clams getting loved on by kids and popping around the 2’6" twice a week and showing some weekends. They have jobs and because of that, they are safe. Perhaps a little rickety fresh out of the stall and possibly dead lame without that $400 farrier job, but safe.
The amount of imported 6-10 year olds I watch sell each year practically haunts me. Where are all the 6-10 year olds we sold 4 years ago? 5 years ago? 8 years ago? What are these horses doing and are they safe?
The whole “the horses have no voice” is defeated the second we throw a leg-over them and teach them not to immediately yeet us into the ground. They do have voices, and good horsemen listen. But there are not acres upon acres of green pastures for allllll of these horses to just retire to at the age of 16 on someone’s dime because taking extensive measures to keep them sound and comfortable is being perceived as abuse. Is it in their best interests? If the alternative is the pipeline then yes, yes it is.