I agree with this to some degree, but it's pretty overstated. I have 14 100's now, and obviously I make a lot of gold with little effort just from treasure missions, but it pales by a factor of 10 the amount of gold I make off AH sales in a month (well over 1M for a full month of active posting)The problem right now is that nothing competes with the mission tables for gold/effort. Your best bet for the 5-7 months remaining in WoD is to level alts to 100. Whatever you could have farmed in the time it takes to level an alt will be surpassed by what that alt generates daily with minimal effort.
Item upgrades sell very well still, especially if you cover all the professions and all the upgrades. I try to keep a stock ratio of 3-1-1 for each type and will generally make 2 sets of upgrades for stuff like BS weapon upgrades compared to armor upgrades.
Epic gems sell very well, and I make 3-4K per gem.
Base level weapons and armor sell decently, especially the weapons.
The BoE's from salvage hut have made me over 1.5M gold in profit this expansion, and I'm really expecting the market to take off at the beginning of Legion with the new system.
Basically, if you have the alts to cover all the trade skills, and you maintain a wide variety of items from them on the AH, use a good add on like TSM to be able to quickly post, cancel, and repost auctions you can still make crazy gold with not that much effort beyond the initial cost of getting a char to 100 w/ a fully functional L3 garrison.