There was a time in Guild Wars 2 that champions were nothing but “annoying”: They were hard hitting trucks with very poor rewards. Because they had poor rewards, most people would simply ignore them and, trying to solo them was really annoying.
Then ArenaNet gave champions better rewards. They now drop a container which can contain crafting materials and gear.
With better rewards, now it’s easier to find people to kill the champions, as everyone get more than simply the satisfaction of killing a very hard mob.
But as they solved one problem, ArenaNet made a second problem — a people problem — appear: People are now teaming up to farm the champions, in cyclical fashion. They are not after the personal satisfaction of killing such small behemots, they are only after their rewards. Queensdale and Frostgorge have notorious farming routes and you can usually find lots of people doing them.
And lots of people complaining when someone breaks the cycle.
It’s not a problem with the game per se, but a problem with our greedy nature: people always want more and more and, with the introduction of ascended crafting and account bound magic find, people need to fill their collectibles storage to level their crafting and hope to get essences of luck to get better magic find.
But the problem is not the greedy nature, or the ascended weapons or account bound magic find; the problem is that with enough people, champion farming is controllable. Sure, they still hit like trucks but with enough rocks, you can easily take a truck out of the road (like, an avalanche of rocks).
If I could point the problem, I’d say that it’s an inherent problem with the event scaling: With more people, the event will pop more mobs or give bosses more health and some new skills, but it won’t scale the event itself.
Let me use two events — used to farm champions — as an example: “Slay the enraged champion cave troll” and “Eliminate the champion bandit lieutenant”. Both events happen near each other and are part of the Queensdale champion farming. Because there are usually 15+ people doing them, they are pretty easy to complete. The troll does a big bleeding damage, but with so many people around, it’s easy to keep healings and condition removals going around; the lieutenant… well, I don’t remember him doing anything really threatening.
The whole problem here is that both events can get easily under control with enough people. And the solution is, simply, add more chaos.
For example: If the cave troll enters a “under control” state, in which the champion/boss itself can’t manage to keep the players on their toes, it should, somehow, bring more chaos to the arena — and giving it more health is not chaos enough still, as it would just take the fight longer and not disarrange the players combat tactic. On the other hand, if the troll got bigger and managed to summon all the grubs that live in the ground of the cave, summon the nearby bears and the nearby spiders and those got enraged… then suddenly the players fight tactic would have to change to include control of the new creatures, better supporting their teammates and so on. Whatever they were doing to control the event would be completely useless at this point.
With the bandits, I can’t see why when realizing he’s outnumbered, the lieutenant can’t just call reinforcements, with the bandits in the whole cave (and entrances) coming back to help him, picking some canons and such.
(And, personally, I think both options add some cool ideas, like the failure — or victory — in such events could trigger some independent events, like the remaining bears go rampaging the bee farm nearby or the bandits start taking points like the farm south and the outpost nearby.)
The whole point is take control of events from players. Let chaos reign.
 Annoying, but not impossible. In the early days of my guardian, I downed a champion yeti and its veteran guards (which kept spawning through the fight). I usually use this anecdote as a point for weapon switching, as I had to switch between my damage weapon (greatsword) and my support weapon (mace and focus).
 I reckon the problem is the detection of “chaos” and “control”. One could argue that a large group of players will bring control to an event, but if you take a large group of low-level characters, you will still have chaos going around; in the opposite side, a small group of players, all level 80 (they would be downscaled, but still…) with top gear and top weapons can pretty much control any fight — see my point above.