Oh, no, that's not what I meant at all! I just meant that if I were to get a horse in their late teens or so, I'd be able to care for it the duration of its life, even after retirement. Same goes for any age- I just used ten as a baseline. I would never give up a horse, or any animal for that matter.
Ok... it's just odd that you use 10 years as a baseline when you're talking about getting a weanling, but maybe I'm reading too much into that. I don't mean to be personal or critical, but I assume when you say you can care for a horse forever, you're saying you have a permanent job, an acreage with a barn and enough pasture for at least two horses (since they don't do well alone) and enough time and money on your hands for daily care plus vet bills, etc.
You may feel we are being blunt, but it's hard to keep reading these posts by people who want to get a baby and raise it when it's clear that it's not a good idea.
I do think you can probably manage a somewhat green broke horse (I would go for a 5-6 year old) as long as it has a good, quiet temperament, and could work with a trainer to further its training. But be prepared to have that horse for 20 some to 30 years! I know a horse that is 38 and still being ridden! There are lots of people riding bitless these days. It's not that hard to transition a horse to bitless as long as they have had excellent training and you're an excellent rider. That's really not an issue.
Also, I totally get what you mean by not wanting a horse primarily for riding, but a) trail riding is very much riding! It requires different skills than arena riding, but skills just the same. For one thing, you do NOT want an unreliable horse when you're miles away from home on a trail somewhere. Things can go badly very quickly. Trail horses must be completely unflappable (or as close to that as possible), must be exposed to a lot of different terrains, situations, etc. You may not care if your horse has dressage moves, but you WILL care if it won't cross water, spooks at a bird or is petrified of cars/ATVs/bicycles, name it. b) Ground manners are just as important as manners under saddle and do require proper training.
Finally, I think there is probably a HUGE difference between raising a mini foal and raising a full-size foal. But I'll let those who have had both fill you in on those differences.