To clarify, AGL is Above Ground Level, or the terrain directly below the aircraft (not buildings, not trees, but terra firma). This means that if you take off, and fly up a canyon embankment till reaching the ridge, though you may be 3000 feet above where you as the operator is stationed, so long as you remain within 400 feet of terra firma you are 400 feet AGL.
Feel free to push the limits, fly it out beyond eyesight, and fly it up to 10,000 feet MSL, bust Alpha if you really want to. Then complain incessantly when the FAA requires a recreational pilots cert as a minimum to operate UASs for hobby or recreation. Many on here who are cautioning against OBVIOUS unsafe ideals are asking you and other pilots rather nicely not to screw everything up. I share information because I want everyone to have the opportunity to enjoy the fun of UASs and flight, I'm a certified commercial heli pilot, I'll most likely lose the ability to operate my UASs last, and probably long after someone screws it up for everyone else.
Don't make a topic that discusses flying out to 20 SM and then come back with any type of comment that pretends that ideal has any fragment of safety in mind. When piloting a manned aircraft, that has a HUGE advantage over any UAS in the case of an emergency, it is difficult enough to select and land a suitable and safe landing area in the case of an emergency, let alone a UAS with a fixed, or slow panning, FPV camera. Your UAS is not communicating with other aircraft, let alone you cannot hear other aircraft out at those distances. Do you have any idea how difficult it is to spot a cessna 152 at a mere 2 miles when flying a manned aircraft at speeds in excess of 120 KIAS, which is slow for aviation, let alone a 2ft inspire 1? How do you plan to use your FPV equipment to clear all airspace around you, how fast can you turn and scan the skies, while flying, while trying to get video, while ensuring you have a safe LZ selected. Even if the title to this thread was in fun and jest you still might incite some other, less well purposed pilot, to try to attempt it because you posted it here.
Look, the FAA is being nice, whether you want to believe it or not. Furthermore the FAA controls the airspace. Let me restate this; the FAA controls airspace. Period. Now if you want to discuss if they should or not, or whatever else, that is fine. It won't do you much good. Regardless of anything the FAA controls airspace and they will have say over UASs. So, bemoan it all, curse it, and fly however you want... and just watch what happens when the FAA has had enough and feels like safety is becoming challenged. Or just be appreciative that, right now, you can fly in the skies without any type of pilot cert, and that given time, and willing safe actions from UAS operators, it will get better for the UAS enthusiast.