I'm not in the habit of serving up advice to writers. But were someone to ask me what subjects I thought novelists should take for their novels, I guess I'd say two things. The first of them would be in harmony with the above-signalled reticence: they should write about whatever they like. The second might inflect this just a bit: they might like to concentrate on subjects they think they can handle well.
Johann Hari has a different view. He wants to narrow the range of fiction. This is not, doubtless, how he himself would characterize what he wants. For he wants 'great realist novels', writers willing to go beyond their personal experience, to act like reporters. He wants Dickens charging out into the 'Great Oven' of the London night, Zola going down the coalmines, Steinbeck living in the squatters' camps... and so forth.
Nothing at all wrong with it. But to lay it down as the optimal or default mode is just narrowing. Literature is about the totality of the world, about all of human experience and then some. To be able to be that it needs every kind of practitioner: those staying home as well as those charging out into the big, wide, thronging world; those who can tell you about the events in one room, about one family, one friendship, one betrayal, as well as those who describe multitudes.