(this is me yelling into the aether hoping someone at Bungie actually sees it)
I understand the reason for not allowing Linux users is because Bungie uses a kernel level anticheat and most Linux distros to not allow applications to access the kernel.
My argument is that this precaution seems useless if it is to prevent cheating because there are other online competitive pvp games that have anticheat detection that are allowed to play on many Linux distros, and my experience was there were more rampant cheaters in Destiny 2 than games like Overwatch, Marvel Rivals, and Halo. (All of which play fine on Linux)
Even now you have a larger cheater issue in Destiny 2 as show in other forum posts. The other previously mentioned games have a much easier report system to report cheaters as well.
My reasoning for not as switching back to Windows is caused by the enshitification that has happen to Windows over the years. With how accessible Linux distros are becoming and how frustrating Windows is becoming for more and more users, you can literally see a steady increase of Linux market share globally. Why not allow that market to spend money and play?
In conclusion, I loved playing Destiny, I haven't got the chance to try Marathon, and I can't play because of Bungie not allowing Linux users for reasoning that doesn't make sense to me.
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Unfortunately as you mentioned Bungie uses their own anti chest that runs off windows. I don't believe that they want to put the funds into updating their kernel to allow Linux.