The vibe I get from your post is that you think Magearna is only ever clicking Trick or Fleur Cannon, and it's possible to play around that? I tend to disagree as one of Magearna's main problem comes from unpredictability. You don't know what you are facing until it uses a move and that can be pretty costly if you predict that wrong. It can start setting up as ur predicting it to be Scarf or Specs, then stuff goes downhill from that. For Choice Specs, going blindly into a defensive Pokemon such as Chansey, Toxapex or Slowbro might result in Magearna locking them for Trick. Furthermore, Magearna's move options on Choice Specs sets are not limited to just clicking the aforementioned moves either, common checks in Excadrill and Ferrothorn are threatened by Aura Sphere while you can also switch out with Volt Switch against offensive checks. Overall it's not as easy to play around it.I did not have problems with the specs set but maybe it is because I play most of the time HO teams. This set is amazing but I would not say broken. I admit that fleur cannon is overpowered and 2HKO most of the meta but in fact it is not a real 2HKO because of the SPA drop. So bring a mon that resists to it and heal back (regenerator mons work too). Also, like all other choice lock item, you become a setup fodder.
Yeah trick is an issue too but in this generation there are no mega and Z move so that move has become very powerful in general.
Choice specs it is a gamble but is it not the case ? I think risks should be rewarded. But today the meta is so fat that we just take a check for the best mons and do not anticipate.
Your statement about Trick is a bit confusing cuz that's exactly what makes Trick good, and shouldn't really be used as a counterargument why Magearna shouldn't be banned. Being able to abuse Trick without any major drawbacks is so good that its another quality of Magearna that makes it super good. Especially against fat teams that u get multiple Trick opportunities.