Any registered user can start a thread about anything they want here. And nobody is being forced to read the threads on subjects that don't interest them. I couldn't tell you what's going on in most of the other threads on this board (let alone the other 3).
There is no "editorial line" at this place any more. There used to be, but that's not been the case for almost a decade now.
Some of us may have "brexit-fatigue", but that's up to you to deal with. Do whatever feels right... ignore it if you want. But don't try to pretend it isn't important. And don't tell people not to discuss important things.
Are there "worse" things going on in the world? Damn straight. How about an ebola outbreak in the middle of one of the current hotspots in the Congolese civil-war? The Chinese "re-education camps" for the Uighur minority? North Korea? What might be about to happen to indigenous groups in Brazil?
Christ, the world is full of awful cruelty and terrible things. On whatever cosmic sliding-scale of human suffering exists, these things are worse than brexit.
But they are also happening to other people. Brexit is happening to us. "That shouldn't matter"? Except it does. It's perfectly reasonable (nay, entirely natural) to focus more of your attention and energy on a problem that (a) directly impacts your family... people you are biologically hard-wired to protect, and (b) feels like you may have some control over it ("a letter-writing campaign to my MP may affect brexit in a tiny way if they change their position... the same amount of effort put into writing about the civil-war in DRC will have zero impact however" is a reasonable position in my view).
But mostly it's (a). I know people who are already in trouble because of brexit. I know people uprooting their entire families because a neighbouring country voted to fuck us over based on lies and criminal activity.
It's the main topic of conversation right now for a reason.