That savings has to be worth the building of expensive infrastructure at altitude, which is unlikely to happen. Air density at 10k feet yields is only 25% less than sea-level, so you're not going to see substantial performance gains. It's much better to focus efforts on choosing a launch location as close to the equator as possible where you can shoot due East (to take advantage of Earth's rotation) without flying over-top of people. This will yield much greater energy savings. There's a good reason why the world's launch locations are where they are.