Not only that but a frame that's not stiff is going to result in more play in the chain and cables that lever against the frame. That means uncommanded or skipp

Not only that but a frame that's not stiff is going to result in more play in the chain and cables that lever against the frame. That means uncommanded or skipped shifts, uneven brake feel, sloppy braking and cornering feel as the frame gets loaded/unloaded in different directions. The real reason a frame should not be optimized entirely for stiffness at the expense of everything else is material fatigue. A frame that maximizes stiffness is a frame that puts its materials under more stress. So if you dial up the stiffness all the way, you'll get less longevity and eventual stress fractures or worse. What most riders think of as stiffness/springiness/feel in their bike comes from the flex (or lack thereof) in their tires, seat, seatpost, spokes, and the harmonic response of their frame to vibrations from the ride. That response is actually a key part of subjective rider satisfaction, I think because we subconsciously feel more in control (because we can feel what the bike is doing better) when the vibrations are more pronounced.