To shoot something down at 70,000 ft (21 km) all you need is a conventional military jet fighter, and a long-range rocket, or even a MiG-31 with a conventional cannon. At best you can make these birds cheaper than the rocket + flight time.

Something that flies at the upper edge of the stratosphere, at 40-50 km (160,000 ft) would be hard to reach with currently available means. You can of course fire a THAAD at it, but you can fire a THAAD at a Starlink satellite as well.