I do have a dismounting problem, though it is partly solved by me and OH too dismounting cavalry style with our left foot in the stirrup. This has often been discussed on NR as a solution for riders who are stiffer or older. See also the note posted above.
But a high cantle has other problems for me - At the time I was going to buy Maisie, I panicked as I knew nothing about saddles. I looked as the RS saddles in which I was comfortable, and Wally and eml pointed me to old style GP saddles. I pasted that advice into Evernote. And the brands I was riding in did indeed turn out to be by the makers they listed!
I didnt take this further as I intended to order a saddle for her to replicate the training saddle worn by lesson horses at our RIs yard. I have a large bottom and it gives me freedom of movement.
But the fact is that I ride regularly on various RS horses both at home and visiting to hack out - and only once on one RS horse have I ever had an uncomfortable saddle with a cantle that pushed me forward. There are lots of oldish saddles in which I am happy, some of them a bit worse for wear in the sense of stitching and patching, but many turn out to be of the brands mentioned in that old thread! The important thing to remember is that a maker may have changed the design over the years even tho the name remains the same.
I believe you can buy such saddles new, or get one made, probably at a price,since the replica I was going to order would have cost a fortune. But there must be lots of second hand, traditional saddles around and if you have a saddler you trust, he or she may find one for you. People who own horses seem to have much more specific saddles these days and even several saddles, to give them the edge when competing.
However, about Dolly and her wide flat back - I cannot ride such horses and you wouldnt be the first person to discover that you need to change the horse rather than the saddle to solve your problem. I hope not, but I know there are horses at both our local schools that I simply cant ride and one of them used to be a favourite ride of mine 10 years ago.