The problem isn't TF by itself it is the entire gearing system in general which has evolved over time to what we have now.
Blizzard themselves have said in various venues that they don't want players looking at stats for gear drops and just look at ILVL.
In their minds, it shouldn't matter what drops you should just be happy for some sort of ILVL increase, no matter how trivial.
With that system in mind, WF/TF are supposed to be bonus "surprises" that give the player a boost ever so often to ILVL.
This philosophy is fundamentally different in many ways, but primarily it goes against the original concept of 'tiered gear'.
Meaning as you played, you had different tiers of gear based on the content you consumed. The formula was generally:
Green gear = easy content, which mostly was questing content.
Blue gear = more difficult content, requiring a group, could be quest content or dungeon content
Blue tier set = hardest dungeons and required defeating certain bosses to get a full set with set bonuses
Purple gear = raid content (most difficult tier of content)
Purple tier set = hardest raid content with tier set bonuses
Orange gear = legendary gear extremely rare and hard to get solo + group content
It was simple and easy to follow even though there was a lot of time gating originally that was relaxed eventually in going from tier to tier.
The new game has squashed all of those tiers basically into one tier: Purples from everything endgame.
Now, everybody is showered with purple gear (or in legion, legendary gear) just from playing normal content.
Because of that there is no sense of tiers of gear in the way that it used to exist. Now the only thing that matters is ilvl progression.
In the past, only the few who were doing the most difficult raid content cared about ilvl progression. Everyone else was moving up the tiers to get to that point. And that is what Blizzard wants. Every body is just looking for a little bit of ilvl progression over time within their purple gear tier.
The other problem with this other than the squashed or totally absent concept of tiers is that Blizzard has removed secondary stat importance. By making secondary stats totally randomized, they are saying that gear is gear and as long as it has a better ilvl you should be happy. But most players don't think that way. Many players still want to stack those traits they feel will be best for how they play the class. And as we have seen since WOD, by pruning and tweaking the classes every expansion along with the effects, this is becoming less useful. So they have removed another way players could customize their class for the build they wanted.
So in this new world of no well defined "tiers" and many different "alternate" forms of content outside raiding TF/WF makes sense.
But in the actual game it really doesn't, not in the current incarnation. Having logical tiers of content does provide the feeling of progression that people need. The current game doesn't provide that. And the systems they use to try and promote that feeling of progression like zone scaling actually do the opposite.
The problem with this is that now everybody plateaus at a certain ilvl fairly quickly and then it is very hard to move up after that.
Even when doing the most difficult content, the increase in ilvl over time is drastically diminished. And god forbid no concept of BiS at the high end or a "tier set" to go with it. TF makes that an impossible goal at endgame.