I think that the worst problems come from average horse owners who don't know what they're doing - they jump their horses too high, on poorly prepared ground, don't condition, and don't understand how to rate a horse between the jumps (that's the number one skill you have to learn). They ride their horses in a couple of shows, enter all the big fence classes, and then wonder why the poor critter pulls up lame. And folks who aren't in that sport think that's a "typical" show jumper.
The real pros aren't that careless. When you spend 100 grand or up on a good show jumper or hunter, you DON'T want him carelessly injured or chronicly lame. If you've put that sort of money into a horse, you hire a professional trainer who sets detailed conditioning and training schedules and watches the horses like a hawk. Horse training has gone high tech, just like track and field or baseball or anything else for humans . . .
My trainer is somewhere on the middle ground. She's got a couple of high dollar horses, a daughter of *Abdullah and two very nice registered Trakehners, but most of her school horses are middle of the road types although they are very good at what they do. She is famous for finding bargains, making the most of her horses and keeping them in splendid shape. Any one of her school horses or ponies can go out for a day with the hounds, keep up reasonably well, and not be completely blown out and soaked with sweat at the end of the day.
I disagree with you about arena versus cross country -- for every hour you're on a cross country course, you had better spend a couple of hours in the ring. The arena is where you hone your technique, make sure you and your horse understand each other, and practice pacing and turning techniques in a controlled environment. You practice there because when you hit an arena jump, it falls down! Most obstacles on a cross country course are built of telephone poles and dirt, and when you hit them they DON'T move.
Also, a good arena is well drained, tilled, and groomed to provide the best possible footing. . . . the ground is softer and more stable than most footing cross country. The worst fall I have taken on Gracie involved an unexpected muddy patch while cantering up a slope out hunting -- her legs shot out from under her and we BOTH went down in a heap! Just had the breath knocked out of us, nobody was hurt, but she's never lost her footing in the ring, even when her toes were a little long.
One thing I've noticed about Western riders is that a lot of the arenas that they use are way smaller than ours -- wouldn't be room for us to negotiate jumping turns at speed in a ring as small as some people use. And tight turns do put a lot of stress on a horse's legs . . .