They are not going to fire either of them - HOTD is still a hot commodity, even after all of this time. As long as he keeps putting out random drawings of the characters from time to time, their publishers can't really complain too much, even if they want to hurry up and get the series over with. Besides, they both have other projects that they are working on - and they are still making their publishers money via those outlets and whatever revenue still comes in from manga purchases and royalties from MadHouse for producing the anime AND that one cheerleader zombie hunter game 'Lollipop Chainsaw'. I bet they got a nice royalty for using the outfits from Saeko and Rei.
Other artists can get steamrolled by their publishers, especially if it's the first couple of serializations that they have done but Satou's been in the industry long enough - he knows how to cover himself.