There hasn't been one media outlet in the UK covering football in the last 30 years that has been able to avoid the basic economics of the market: the vast majority of customers support United, Liverpool and (to a lesser extent) Arsenal, so the content must reflect that to maximise profit.
From that point ensues a downward spiral, and you end up with (e.g.) the BBC putting out more coverage of Klopp's Joey Tribiani impressions than transfer window activity.
I can't see that being any different for the Athletic.
I guess it depends though since it seems the intent is for the Athletic to cover more than just football. You shouldn’t then have to cater to the popular teams readers because losing some of them is outweighed by fans of other sports signing up.