Absolutely the game changes have been changing the population demographics over the last few expansions. I do not believe any of the people I played with through WoTLK, Cata and MoP would enjoy the state of the game now. Some times they pop in for a week or so, but they always leave.
The current game play, and the feel are just too different for them to adjust. Plus they can’t find people “like them” anymore. Those players are just gone.
As your friend said, maybe 10% did the hardest content back in the day. I would guess you could make the argument that it’s flipped now. I definitely feel like I’m in the 10% that isn’t chasing the meta right now. I’m off goofing around, doing pet battles, casually leveling alts and… posting on the forums. I only have two chars at max level. A horde Shaman and Alliance Paladin. Content I have “done”: A few normal dungeons, Torghast a few times and a few world quests. I just picked up my first memory for the rune carver this week. Maybe in a month I’ll have enough soul ash to craft the Legendary? Not sure.
Edit - I forgot have 3 chars at level. I leveled a Horde Paladin doing pet battles. Kinda funny hitting max level at an iLvl of ~66 lol.