You're right, I actually hadn't checked because I assumed that would be silly to have Head/Shoulder/Chest on every boss in the raid. With that said, it's still entirely possible to not get an azerite piece, especially if you're getting other items. It's just luck of the draw which you get.
That is where you're wrong. Master Loot allowed you to trade everything with no conditions.
The scenario where that matters is in guilds. I will not be able to trade something that the game qualifies as an "upgrade" even if it's not really an upgrade, just higher ilvl. Guilds are teams. It's not as selfish as pugs. I would much rather trade an "upgrade" if it's more useful to somebody else. As a simple example: If I'm wearing a 410 piece and I get a 415 the upgrade is tiny for me (or none at all depending on the secondary stats), but maybe one of my guildies is wearing 385 on that slot. I would trade that instantly, but personal loot does not allow that to happen.
And it is not a separate feature because personal loot is now enforced. We can no longer choose to trade "upgrades".
I don't see the point of comparing first 3 mythic bosses number of players, to number of players that finished a +15. In both cases you are looking at an extremely narrow subset of the entire player base.
A better perspective would be how many players have done M+2 or higher. For purposes of an example, I took the 5% number (of frost mages) and multiplied by 20 to reach 1,277,400 unique players completed a +2 or higher. Using your 35k number, that gives us 2.75% of the player base completed a +15 or higher.
What you are trying to tell us all is that +15 is easy, yet less than 5% of the player base can do so. I could get on board with your assertion if 51% of the player base was completing +15.
It's been pretty disheartening in prog to see people get loot they don't need but it's bis for you but they can't trade it because item level penalty. They need to change this.
Because that's the amount of players who participate in relevant PvE content at a reasonable level. The first three bosses are really easy even for low-end guilds so it makes for a great comparison. Of course similar to mythic raid bosses, +15s are not easy in the sense that someone who has never played the game before could do them effortlessly.
The original point of the discussion was about the residuum prices for 415 pieces. Someone who has once done a +2 dungeon (tuned for ~350 ilvl) shouldn't even dream about getting mythic raid level gear without putting in more effort. +15s or +17s are not supposed to be easy for green geared LFR players. They're easy for your average mythic raider.