What will ultimately limit SpaceX, and launch in general, is deposition of water (either water in the rocket exhaust, or water from later oxidation of unburned

What will ultimately limit SpaceX, and launch in general, is deposition of water (either water in the rocket exhaust, or water from later oxidation of unburned fuel in the rocket exhaust) in the upper atmosphere. The stratosphere and mesosphere are extremely dry, so this limit is lower than you might think.