I assume everyone in the final season could see the writing was on the wall in regards of Dallas's future. I guess Sheree just thought that even if the show somehow had gotten to a season 15, it would've been further cut down in budget and would've ended sooner rather than later. But I think everyone could probably feel the impending doom on the set.
If Dallas had been in a healthier shape I'm sure Wilson would've just asked for a regular maternity leave/accommodation rather than getting off the show permanently. Of course she couldn't have seen the movies or the reboot series coming.
It's hard to know. I do know that as early as 1988, Patrick Duffy said he couldn't see DALLAS running passed 1991 -- which I assume was when everybody's contracts were up... Even before the '90/'91 season premiered, the press and people close to the show were talking as if was "understood" that this was going to be last season.
Had there been another year, I'm sure Sheree would not have been written out had April not already died -- Katzman really "liked" Wilson, so her job was safe if she wanted to continue on with the show -- as much as some of us hated her being shoe-horned in to being Bobby's wife (and they had to change her character to do it).
I would have turned her into one of Bobby's female stalkers who followed him to Paris when he married Tracey McKay, both women mowed down by NATO or Isis or the MI-6 or the Moldavian rebels or whoever the hell did it.
I liked gold-digging April, but her shrill wife-mate status was forced and a mistake.