I was having this conversation with a friend the other day...obviously a mare that produces GP level horses and/or stallion offspring is a successful broodmare. But, let's face it few horses make it to GP (for many reasons...talent, rider, injury, etc.) so hence few mares are producing GP offspring and even less males get to stay a stallion. So what do you use as a gauge of a successful broodmare? Offspring competing Third Level and above? Only if offspring at FEI level? Only if daughters are EM/SPS?
There are lots of ways to qualify "success" of a broodmare, just as you pointed out, i.e. Solid dam tail line with proven athletes, Premium producers, competing offspring, winner offspring, approved sons, elite daughters and so on. I think it's somewhat like a revolving door or a moving target for that reason, so at risk of sounding evasive, my motto is: "Pursue Excellence - Ignore Success" - Deepak Chopra
For me, the answer lies in how what I'm producing matches up to my own compass. You can't rely 100% on inspection scores, breed show results, Elite/Keur titles, or even the results a mare's offspring have in sport. Yes, these are good things, but they are also kind of the icing on the cake. A good trainer can take a decent horse and get it up the levels. Sometimes you have a good day at a breed show or inspection. Some mares aren't properly prepared or are injured, so miss out on becoming Elite.
When I consider success, I ask myself these questions:
Have I started with the best mare for my goal?
Am I expecting the stallion to "fix" too many things?
Is the foal what I hoped for?
Is the foal an improvement?
In other words, have I made a good match, one that is both a general improvement for the sport and one that I'm happy with?
The best horses I've produced have been out of stellar mares with little need for the stallion to change a thing.
Because I'm not a breeder but that I focus on performance jumper, I give a lot of importance to the mare.
In order to make my choice when choosing a prospect, the first thing I will look at is: what his family has produced, from a sport point of view because I don't care if the line has produced approved stallions, the "modern look" or any other beauty elements, only performance in the arena matters to me.
Only then I will look at the horse in front of me and will pay attention at conformation flaws that can impact soundness, again I will not give importance to other negative conformation elements if the horse is capable to overcome them.
It's clear that a breeder need to have a very different set of important parameter when he look for a breeding mare.
Different goal, different horse.
"If you can meet with Triumph and Disaster, And treat those two imposters just the same"
Rudyard Kipling Quartz Rouge
To me, a "successful" broodmare is one who outproduces herself consistently, conceives easily and repeatedly. As far as performance goes, it's been my experience (especially here in America) that a vast majority of the horses are held up because of RIDER short-coming. If every horse was ridden by Steffan Peters, or Carl Hester or Ed Gal, most of them could perform at GP level, even if the scores were modest.
So I can't hold it against the mare because her foal didn't go to an accomplished rider. As long as the horse stays sound ( a BIG issue) and succeeds at the level his/her rider can achieve, then the mare that produced that foal is "successful" (assuming the other criteria is met).
Obviously a mare that consistently produces stallion candidates or Premium Mares should be considered successful -- those horses are evaluated very strictly and should be of a higher quality than the "average" horse.
But if I was to consider a mare, I would look at all these things and "judge" her within the confines of her opportunities. Sometimes one breeder will not really "get" a broodmare in terms of what sort of stallion suits her best.
Then another breeder gets her, and tries her with a whole different sort of stallion and gets much better results.