Other relevant context: - evolutionary design approaches already result in aircraft ~1% more efficient every year, with much of the R&D offloaded to the engine

Other relevant context: - evolutionary design approaches already result in aircraft ~1% more efficient every year, with much of the R&D offloaded to the engine OEMs - clean sheet designs have to save enough in fuel vs evolutionary ones to offset the additional operational costs of operating a brand new aircraft type as opposed to a variant model. Airlines like having lots of the same kind of aircraft (and nobody has the production capacity to replace an entire fleet at once) - clean sheet designs are much more likely to have significant delays, groundings and safety concerns. For an airline, this diminishes the value of the planned-to-be-more-efficient aircraft. For the OEM, it's a massive reputational risk, and if you screw up the clean sheet design badly enough your competitor captures the entire narrowbody market. Boeing lost enormous market share for a patchable software bug in a direct descendent of the most successful aircraft ever; now imagine a "clean slate" which doesn't deliver on its fuel economy promises and has unforeseen issues with the long term structural stability of its wings vs Airbus cheaply updating its established design - if the project is wildly successful, it's not all good news: you've wrecked the expected commercial lifespan and residual values of existing aircraft models, which doesn't please your customers or your aircraft leasing subsidiary. Also, your competitor learns from your successes and your failures when launching their "clean sheet" aircraft which has lower research costs, a shorter time to market and is more mature when it does launch