The issue of pushing horses into sports is an interesting one - it's one I've had a few times lately (before the OT business kicked off!).
It's complicated because horses are designed to work - that is, they're designed to cover large distances every day. They're not designed to carry people at all - there are significant biomechanic consequences. So the extreme answer is that we shouldn't have them at all if we insist on riding them, and (/or) keeping them in relatively confined spaces...
Except that horses live longer (on average) in captivity, than in the wild - so perhaps the benefits outweigh the downsides, and, by extension, it's arguably ok to keep them. But then how much do you work them - how far is it ok to say "I want you to let me ride you and work you" - given that the horse, by choice, would mostly rather be in the field eating. Leaving the horse in the field to eat - according to its basic desire, you dramatically increase the risks of obesity and related conditions, metabolic issues, laminitis - hugely deleterious for the welfare of the horse.
But if you work it, sooner or later, you'll have to apply some pressure - to respond when you say "stop here" but the pony wants to keep going, or you ask it to go past the scary car, which the pony thinks is going to eat it. You could back down every time - but the risk of ending up with a dangerous animal (or, with a native pony-type, just an overweight, lazy one!) dramatically increases.
Even those who practice "soft" training methods put pressure on their horses - some openly, some less openly, some apply physical pressure, some psychological. I'm of the opinion that the psychological pressure is often worse for the horse than the physical - but that's a matter of opinion. The Parellis and Monty Roberts are relevant here - at various times, these have been considered the softer, natural approaches, but are, in reality, anything but.
There isn't really a clear answer to what's ideal in managing horses. Quiet hacks are lovely, on a nice day - but it's just another type of risk, looking at the roads today and the number of horses hurt on them - and how far is that work sufficient for a horse (for some, it's plenty, for others, it's not enough to manage their weight without starving them!).
Certainly, the human desire to push them to the extremes of their capabilities in sport isn't ideal for welfare - the whole concept carries inherent danger to the horse - huge, solid xc fences, hideous dressage training practices... But are there similar risks, similar injuries at lower levels, due to rider competence, or horse suitability issues? Where do you draw the line on what's an acceptable risk to the horse, and what's acceptable pressure to apply on a horse, for its own sake?
These are all hugely interesting questions, and issues facing horse owners in the modern world. We have huge problems with the movement towards leisure horses, who work less and are often fed more (feed company marketing!) and stabled too much or grazed on rich pastures. If I were feeling contentious, I might suggest that "kindness" to horses is the biggest welfare problem in the UK right now - but I don't want people to think I mean that cruelty, or unkindness would be preferable. All I mean is that there is a middle ground - that sometimes pushing a horse to do something it's not too keen on, or is approaching its limits, isn't the worst thing you can do.
That's not to say that I'm defending over-use of the whip in any circumstance. I'm defending the concept of carrying one, the concept of appropriately using one - or, indeed, any other means of applying pressure - legs, spurs, bits, bitless, carrot sticks, psychological pressures - all of which are just different tools for the same job - getting the horse to do something we want it to, and it doesn't.