Honestly there's a pretty obvious explanation for why Wrath-;arts of Pandaria all felt like your raid was gaining power over time and why Legion and BfA doesn't... It's titanforging mostly but also combined with mythic plus as a constantly repeatable source of said titanforged gear.
In past expansions, the best gear came from the raids. The gear that dropped from finishing a boss was the only thing that was ever going to drop of that power level. Over time, your guild would slowly start to accumulate their BiS pieces off of the bosses you were killing allowing you to down new bosses for new BiS gear. Each week of a new raid, SOMEBODY was getting SOMETHING relevant to the powerlevel of your entire raid even if it wasn't a BiS piece. Maybe your Fury warrior got a new 2 handed weapon that wasn't BiS but was better than the BiS from the last raid. Maybe you had a really powerful trinket drop and your tank was now godly.
But now you aren't guaranteed to keep getting better gear. I don't know how bad it is in BfA, but I know there were several players in my guilds in Legion who would walk into Mythic difficulty wearing items that were nearly identical in power to the baseline mythic gear. We did actually kill bosses for the very first time on a new difficulty without a single player getting an upgrade. They maybe already had a titanforged piece of gear or a warforged piece that rolled with a socket and was better than the default mythic pieces of gear.
Or you could have a BiS piece drop for your raid, but the useless piece nobody needs might have titanforged instead of it. It's even worse now that masterlooter is gone. You can't funnel specific pieces to the right players anymore, and item level "upgrades" might not always be an actual upgrade for the player or your raid. Maybe you get (and this is just random numbers) a piece of 230 gear and you have a piece of 220. Great! It's an upgrade!... but the stats are kinda bad and you have another player in your raid who is using a 190. You'd rather have given that piece of gear to him to catch him up, but you can't trade it to him because you don't have an item level equal or greater to the gear you just got.
That's a situation that would come up in raiding guilds that even casual groups would have been able to identify and handle without there being some dumb drama.
It seems to me that they are trying to balance their game not by putting in additional time and effort, but by removing the player's ability to make choices. They've been doing it since Cataclysm honestly. Removal and dumbing down of talent trees, forcing you to wear specific armor types or take a penalty to your main stat, removal of hit and expertise, removal of reforging (which is also a mess combined with the last point**** see below), trying to make each secondary stat functionally identical to the others for every single class, adding RNG to gear so you can't make a BiS list, and now the removal of masterloot.
I honestly have no idea who they're balancing this game for anymore. They claim Titanforging was added as a benefit to casual players so that you could get an exciting upgrade from anywhere and not just from raiding. I don't really think that's true. I think they put it in the game as a way to incentivize the hardcore players to play more. They would clear the raid in less than 2-3 months, get BiS in another month on their mains, and then possibly unsub or log out for the next several months while they waited for the PTR to come out with raid testing. In fact, titanforging in it's initial implementation was actually CITED to be for this very reason. They wanted to give raiders a reason to feel good about reclearing bosses that they had already beaten.
It's crazy that it's come to these conspiracy theories, but honestly I think Blizzard has put in titanforging and removed Masterloot under the guise of being a benefit to the casual player when it's actually designed to force the hardcore players to play longer. "But why would they lie about it? That wouldn't make any seeeeense?!" says the casual player not being affected by the changes in any negative way. Well sir, it's something that happens all of the time in the real world. If they put in a mechanic that only punishes hardcore players, hardcore players will complain, nobody will argue against them, and Blizzard will look stupid if they don't revert the changes. By claiming the change was made to BENEFIT casuals with the unfortunate side effect of hurting hardcore players INSTEAD of the change being made to hurt hardcore players with the added benefit of also sometimes maybe helping casual players, you're now creating an "us vs them" mentality every time somebody who is hardcore speaks out against it.
***The whole reforging situation was handled with one of the dumbest arguments I've ever heard. They claim to have removed reforging because it didn't feel fun needing to go back to town to reforge all your gear for the optimal hit and expertise caps before you could wear it... and then also they got rid of hit and expertise in the SAME EXPANSION. It didn't make any sense to anybody.
Wow. Long post. Lots of anger and resentment built up. These 26 minute AV queues in Classic WoW really give me time to rant.