I think that while writers may indicate they would like to get so and so back, it is more the suits who say they want to bring an actor/character back. Currently, Days is on a kick of bringing back fav characters/actors for short stays. Writers are told the show wants so and so back, etc. and story is written.
But when it comes to newer characters, story is written or outlined, presented, and so the call goes out to fill roles. From what I understand, Story outlines are written up to a year in advance, with the scripts being written nearly 6 mos. in advance. Thus they can tinker, etc. Once everything gets on film, it is the feedback from the viewers, (or observations from the producers/directors/etc. ) that determine the fate of a character or actor. And then, there is the budget, and how much room the show has for x amount of cast.
I believe it is the suits who decide to not renew a contract, or to suddenly let an actor go. It is ultimately THEIR call.
This all brings to mind the infamous Salem Stalker storyline, in which MAJOR characters were being killed off right and left, 10 in all, including Alice. As the feedback began, (which cannot come until shows are airing, and that is months after filming) the suits were horrified. All the actors whose characters were killed were fired, were out seeking other jobs, but the suits ORDERED the writer (Reilly) to "fix" it, bring them back, they did not care how, or how unbelievable it might seem, GET THEM BACK. So, we got Melaswen. LOL