I think treeless is mostly a gimmick. I think flex trees have their place, if you understand what they do. I'll offer my THEORY on what they do, although I'm not a saddle maker:
50 years ago, a tree was made of wood and leather. The wood was pine because pine flexed a little with the movement of the horse. Not a lot, but it wasn't as heavy and rigid as oak would have been. The leather also had some give, but it wasn't as strong and weather proof as fiberglass.
When fiberglass wrapping became the norm, the trees became more rigid. Again, we aren't talking about inches of movement, just a little, small fraction of an inch give under the pressure of riding. That is strong, and it works fine for most horses & riders. But with time, there has been increasing emphasis on horse 'sports'. Instead of trail riding being the norm, it is becoming the norm to ride in an arena and to repeat the same movements hundreds of times in an hour to train the horse for a sport. And a saddle that fits OK for 4 hours of trail riding may not work so well for a horse training in 'western dressage' or doing a barrel pattern again & again, or using dressage to get the horse in shape for something else.
I think the tiny flex allowed by the new flex saddles is an attempt to get back to what the saddle would do when it was pine & leather. It allows a manufacturer to use a ralide tree or a wood & fiberglass tree that performs like the old saddles made by hand did. At least, in theory.
Unless someone is competing in sports and riding the horse all the time (as in 6 times/week in intensive training), I think it is overrated. I feel like I do when I see the constant response to 'get a saddle fitter'. Hmm...what did folks do for the hundreds of years before there were saddle fitters, or those of us who live hundreds of miles from the nearest saddle fitter? A poor fitting saddle can screw up a horse. My Appy has white spots on his shoulders and a scar on his withers where a ranch used him with a too wide saddle. But it isn't rocket science. By breeding he is 3/4 Arabian and has high withers, and a wide full QH bar saddle didn't fit...golly! That would take about 5 seconds to figure out!
I wouldn't pay more for a flex tree saddle, but I wouldn't reject one, either. I doubt most horses ridden by most riders can tell the difference. I would NOT buy a treeless saddle, because I've carried backpacks with frames, and some without, and MY back prefers a frame to support and distribute the weight!
All just IMHO. I took up riding 4 years ago, and spent too much time trying to find the "Magic Saddle" that would improve my riding. I'm also still looking for the "Magic Bit"! I settled on an Aussie-style saddle and a single joint snaffle as best for me & my horse. How I ride has far more effect than my saddle, and since your horse is doing fine, I'd guess your riding is OK too.
Although if I could find a barrel racing saddle that fit her short back, I wouldn't mind switching to a western saddle...but it isn't in my budget right now. It seems the best saddle is the one you want to buy next!