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@gbj_tester
The "triangle" is more like a tetrahedron, and the loading is not in line with the seat stays*. Therefore, they bend. Some manufacturers pre-curve them to make them bend more, and in a particular way, but even straight stays bend.
If you want comfort, though fitting bigger tyres is more effective than any tweaks to 'rigid' frame designs.
*you could make a frame where the seat stays only ever saw a pure compression load, but it would have to have spherical bearings at each end of each stay, accurately centred on the centreline of the stay.