Launching at 60k feet doesn't buy you many advantages except there's less atmospheric drag up there - it's about the speed, not so much the altitude. But you'd

Launching at 60k feet doesn't buy you many advantages except there's less atmospheric drag up there - it's about the speed, not so much the altitude. But you'd gain the problem of hauling a ~500 ton rocket up to 60k feet, and you'd anyway have to launch that plane from ground zero.