BBM
If she said that NG was going to be her last public appearance, there's only a few ways I can interpret that:
1) LE has told her they are fairly certain they are close to finding Sierra's body, so there will soon be no further need for media publicity.
or
2) It could be that she finds media appearances so emotionally destructive that she and Steve have agreed that Steve will take over the public appearances until Sierra is found.
From other cases of missing children, I consider 2) to be unlikely. It seems like most families of the missing would be more than willing to crawl on their hands and knees over broken glass facing their worst nightmare images to get publicity for the plight of their missing loved one.
I mean, honestly--if families weren't so desperate, why would anyone appear on NG and take the (high) risk of being ripped to shreds for some little thing NG thinks they did wrong? The families appear because they are willing to do anything.