Gary Powers was shot down in a U2 at 70,000 ft in 1960. They thought that couldn't be shot down by a surface to air missile too.
49 years later, the max altitude of a surface to air missile is 5,000 ft less? I don't think so.
Balloons can go way higher than 65,000 feet, the record is 53Km, i.e. more like 150,000ft. Put it up there and it might be safe