I think DJI put the hard limit of 500 M because the battery time used to get there and down again does not leave much for flying around, especially if high winds are encountered at that height.
well, it is not exactly true and has nothing to do with technical. a 4S lipo powered drone, at the comparable AUW and prop size should be capable doing 15m/s ascent speed consuming about of 25-30A. to descent it takes much less - 5-8A top. so, we have here a 3850 mAh lipo, if we want to burn 2800mAh at, say, 28A rate then we will have about of 6 minutes to do so, give or take, and at the 15m/s rate it should be almost 5km up in the sky.
now, my fastest racer does 40m/s and it burns 120A at doing it. so it can go, well, quite a bit up quite fast, until battery lasts.
can a mavic do 15m/s ascent? no clue. i saw somebody posting about settings for the smart mode up speed at 10, i left mine at 8, as it is more than enough for me. even with that it should be able to climb quite high, if needed.
i hope it will remain locked, as, in my observation, the amount of effort that is needed either to build a custom drone, or to hack, correctly, a commercial one presumes that person should possess sufficient amount of intellect to know what is safe to do and what is not. there are exceptions from the rule, of course, but, i think for most of people buying those drones at the store it stands correct.