I get the complaints about the randomness for sure, but I'm fairly convinced that if they removed the randomness people would complain about lack of content instead, since the randomness vastly increases the amount of content a player can do and still have a chance at rewards. Or if they went fully extreme with the systems, people would complain that they "have" to do content non stop.
For example, take the logical extreme opposite of what we have now. Everything you do gives you currency, harder content gives more. You use that currency to buy gear and legendaries. If there is no cap on how much you can earn, people rage that they "have" to run content non stop until they get their BiS legendaries and best gear, then suddenly they complain that there is nothing to do on their main and they have no reason to log in for raids, and raid rosters start to lose members. This system is much less random, but I don't think it is better in any way at all.
Now lets take a middle ground, everything is the same but remove WF/TF completely, and you earn currency from activities that lets you buy legendaries. Again, people complain that they have to run content non stop until they get their BiS legendaries, but that isn't as big a deal here. The big problem is that someone who raided the previous tier will have easily gotten their full BiS from raiding. Then, when the next tier hits, they log in for raids and nothing else, (because without WF/TF none of the Mythic+ rewards are an upgrade except for the weekly chest) complaining that there is no content to do besides raiding. Maybe that patch introduces a new legendary to grind out, which they do a lot of content to get, then go back to only logging in for raids. They get their BiS from raids and suddenly stop logging in because there is no content that can give them anything, and they complain about lack of content. Is this system better? Maybe, there can actually be an argument here, but it definitely still has problems.
As much as WF/TF is annoying, I know the struggles of leading a moderately top tier guild (my guild killed heroic LK pretty early on back in WotLK), where when people are "finished" gearing up they start to disappear. Many people dislike the WF/TF system, but my personal experience is that burnout from randomness causes less turnover than quitting from having nothing to do and no way to progress. Was the same thing in PvP, you could buy all your gear as you got points. I had a group of ~6 throughout WoD that I did arenas with and the trend was always the same, everyone logged in every week while farming enough conq for weapons, then started logging in less and less after getting weapons, and most stopped logging in at all after finishing that season's set. Only to start back up again next PvP season.
To be clear: Mostly just playing devil's advocate, there is definitely too much randomness in Legion. Biggest offender is the legendary system, something HAS to be done, it is FAR too random and FAR too punishing. But I think some inherent low levels of randomness aren't bad, and I think WF/TF are fine, if not healthy for the game. But overall even that could benefit from a little de-randomizing, without going fully deterministic.
