I don't believe I said it was impossible, especially if you're riding many of the same type of horses, or horses with similarly shaped backs. A "medium" tree in most saddles will fit many "English" type horses that are TB or TB-types. A "wide" tree in most saddles will fit many wider stock-type horses or warmblood types.
Courbette's Swiss E-Motion tree self adusts to most horses that fall into the medium-narrow to medium-wide category. They only don't fit broad backed horses, like thick warmbloods or flat backed horses. You can often find these saddles reasonably priced used on ebay
, tacktrader, or consignment shops. You can see the line of Swiss saddles on Courbette's web site: Courbette Saddlery Company, Inc. Online Catalog
Their Magic saddles are reasonably priced new and very hard-wearing.