Yes, you just defined what a softtail is! It's not so unorthodox in fact. The Cannondale Scalpel has carbon fiber flexible chainstays and is one of the winningest race bikes in history. If only they'd make a 29er version.
The progressive travel of a softtail is one of its best qualities. It just naturally gets firmer the further down the travel you get, vs. a full-suspension bike which needs engineering and a good shock to do the same.
There are "pseudo-softtails" out now like the Salsa Mamasita, Cannondale Flash, and even Specialized Roubaix road bike that have flexible carbon fiber chainstays and seatstays, but no shock.
The Dos Niner is not the most durable frame, it's made of scandium-aluminum and the tubing is very thin and prone to dents. However, Salsa is a good company to deal with on warranty issues, the price is excellent, and at less than 4.5 pounds, it can be built up very light.
kstang wrote:
To learn I did a quick google, and from my search it does not look like that frame has a pivot, So do the chain stays flex where they meet the bottom bracket? If so what is the fatigue failure rate? Its an interesting idea, but the idea of a controlled flex like a leaf spring on a car.