Halt and Catch Fire and also Turn Washington Spies have been very very lucky to of lasted as long as they have. They are poorly rated, barely make a mark internationally and do not have the critical acclaim and award nominations to back it up like other low rated shows do. At least fans have been lucky to have both shows conclude with a final season next year.
I was reading somewhere earlier that networks can afford to keep some low rated shows going if they already have big hits on their network already. So for example, as long as AMC are still making big bucks from the likes of Walking dead and Better call saul, they can still keep poor rated shows like Halt and catch fire going.