Cost is not absolute
My used composite boats cost me less than comparable plastic boats. Given the choice between new plastic and used composite, I’ll take the latter unless there is a specific reason to go with plastic, such as for a “coaching” boat that’s going to be severely abused.
That’s not absolutely true
Baidarkas were intentionally designed to flex along their length and their efficiency is on par with modern plastic and composite boats. Stiff is not always better; it varies with the conditions you’re in.
I don’t see how flex in a kayak could contribute to propulsion, as it is predominantly in a vertical plane, roughly perpendicular to the direction of travel. That doesn’t mean that it has any negative effect, as it’s not absorbing propulsive forces, either.