Just my opinion on this subject since everyone is giving one :)
Why not make the qualification based strictly on the performance of the horse/rider team's performance during the year. Take the top ? percentage of each weight division or region for that year. Have the Championship Ride late in the ride season or in December after year end points are final. The horse/rider teams that have the least number of miles have probably performed the best at the races they have attended. In other words they have probably been racing all year and placing high (and yes....completing and staying sound). Those that have high miles and have chosen to ride more conservatively will still qualify. Those that have high miles and high placing will definitely qualify. Those that bought a "made" horse for that year, still had to race more than one or two races on that horse to place in their weight division/region. What is the least number of miles this year that a horse has to place in their weight division....around 300+ hey, that's 6 50's or 3 100's for that horse/rider in a season. It's not like they bought the horse just for the championship ride. The bottom line is, regardless of how the teams did it, those that place in their weight division or region have performed the best during that ride season. The "best" should be the teams that compete against each other to determine who is "champion". ( In my opinion, of course)
I don't think the qualifications for a championship ride should be the "watchdog" for horse safety. The qualifications should be in place only to distinguish who is the best horse/rider team for that year. Veterinary controls, rider responsibility, and AERC rules should be the watchdog. I am not going to judge someone's conditioning and racing program unless I know for a fact what they have done. Some people CHOOSE to enter only one or two races a year yet they may have a better conditioning program than someone who has 1000 AERC ride miles that year. The proof of their program is their success rate - if they can place high in their weight division or region with few races that is enough proof for me that they DESERVE to enter the championship for that year.
AERC has an excellent education program, mentors for new riders, more seminars, vets that stress "Rider Responsibility for their horse" and the highest veterinary standards in the history of this sport. As a whole, endurance riders are probably the "horse knowledgeable" of all equine competitors. I am willing to give the benefit of the doubt to those who choose to attend a "championship" with less "ride" experience. Those that choose to be ignorant will probably ruin horses and hopefully not be in this sport long. I don't think trying to "legislate" them out of a Championship Ride is going to save their horse or make them more caring about their animal. They will just thrash the horse at the next non championship ride. At least at the Championship Ride you know the highest vet standards will hopefully eliminate them before there is a problem.
If we want to reward longevity, eliminate a one day championship ride and reward the championship by either the most lifetime miles accumulated by any horse ridden that year or by the high mileage horse that year (I think we already have that award) or take a % of the highest mileage horses for the year and let them race against each other in a head to head competition. Now that rewards longevity :)