I know not everyone will agree with this, but in my mind, arena work is in preparation for riding out in the real world. A safe place to ride and train while you and your horse figure each other out. And maybe for shows, just so you keep everyone in a nice contained space.
But to have a horse that never rides outside an arena? That's like having a car that never leaves the parking lot. Sure, you can drive around the parking lot for a while, but don't you ever want to get out on the roads and drive around?
To me, trail riding is the "real world" and an arena is a place to train (and show if you are into that). But really, back in the old days, people rode horses out in the real world. They transported people, carried them into battle, got them across the country, carried goods. Horses are MADE to live out in the world. Even in their natural state, they are out in the real world. So to shelter them in an arena their whole lives seems pretty sad to me. It's like you have created agoraphobic horses that are afraid to leave their safe little confines because they have never been exposed to the real world.
Are there some horses that are better than others at trail riding? Sure. Maybe a very select few really would never make good trail horses no matter what. But I would hope we wouldn't be selecting for horses that couldn't make it out in the real world. They should be the exception not the rule.
I have a 3 yr old gelding. I love him to death....he was born from a mare I bought to trail ride (a buy-one-get-one-free deal
) and I am just now getting him out on the trails. He already spooked and dumped me once. But he is making steady progress. I hope and pray that he will be the trail horse I want him to be because honestly I don't know what I would do with him if he didn't trail ride. I don't have an arena and I'm not going to build one just for him. Not only would it be too costly, but what good is a horse I can't ride outside? I don't want to ride inside a fence forever. There is a whole big world out there and I want to see it.
So yeah, my baby better make a trail horse or I won't know what to do with him.
Now the example like in Japan I can understand. They simply have no place else to ride. So it's either an arena or not riding at all. In that case I would choose an arena too.
Back when I was a kid I thought all horses could trail ride, and then if they were really good they also did shows. Now I realize showing is totally different and a horse trained to show is not automatically a trail horse.
I had a friend with a rental stable some years back and she used to tell me the most nervous riders she got were dressage riders.....because they weren't comfortable outside an arena. I was sort of dumb-founded at that, because in my mind they should be superior riders.....right? I can count the number of riding lessons I've had on one hand and I ride outside an area all the time. So that is just weird to me! (Just a note, I'm sure not all dressage riders fit this example, this is just what my friend told me in her experience).