I make about 60% of my profit from old world item sales, and 40% from BFA sales. What BFA provides me that old world items don't is immediate gratification- If someone posts a stack of 200 monelite ore 10% below market value, I can immediately buy and repost for 2k profit and have it sell in 2 minutes, and do this 20 times an hour. I currently have 2000 pieces of ghost iron sitting on the AH that aren't selling, and since I put it up I've sold plenty of flipped BFA materials. Ghost iron absolutely sells at a high rate, but it is slower than BFA mats.
"The way the glyph market works on most realms is that there are a couple of people willing to put in the long hours who will have every available glyph posted profitably, and undercut within minutes of being undercut. They all reduce the price very minimally when undercutting, and the "competition" isn't about price, it's about who can relist more frequently.
What’s worse, it’s rarely satisfying when you do make it to a new milestone in your grind to unlock the gear you need. Legion had legendary items that changed the way an ability functioned, which would also completely change the way classes played; the Azerite traits in the Battle for Azeroth are rarely as interesting. Instead, they are simply passive bonuses that, in most cases, help you do more damage without any extra effort on your part. While some classes are lucky enough to have traits that change their gameplay slightly, Blizzard has so far seemed to favor nerfing those traits out of viability for most specs.

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I think that's server dependant. I make a lot of Golems, but the price on my server is high enough that you could buy all the living steel and still make about 20K (almost 200kg on AH). Its just that you proc all the way up the chain. You proc 20% of your trillium, and then 20% of your living steel. SoH and Ghost Iron are such low effort farms, too. 20% of 180 Trillium Bars at 500g is 18k more profit, 20% of 150 Living Steel at 3500g is another 105kg. That's an extra 25k profit per Golem. If they stop selling stockpile them for the next expansion, they always boom with the price of herbs. And a consistent 20% proc rate is the opposite of RNG, its extra free stuff as a benefit to completing an extra quest (not to mention for new alchemists the investment of leveling Vanilla Alchemy). You're looking for some get rich quick scheme, where the real way to build wealth is patience and volume. No market is going to be viable because you want it to be. Only because you make it viable.