I think Wilson would have to have three or four bad outings strung together for the Giants to go to someone else. Last ST, Wilson was considered a pretty serious contender for the closer spot but was extremely wild, probably due to nerves/trying too hard in spring. Benitez eventually won the job, and Giants fans know how that turned out...(culminating in two consecutive balks leading to a BS)
Anyway, I think management will be much more lenient and slow to give the job to someone else, mainly due to the fact that this year has pretty much been given up on. I don't know if Walker would be the next one in line, they tried him out in 2005 and ended up trading him to the Rays and then getting him back last year. I think Walker is much more comfortable in the set-up role, and he has thrived in ST.