Out of their 21 scripted shows it's currently sitting at #5 in the 18/49 demo & #13 in viewership numbers. It's only slightly down from last season's numbers so unless Seth plans a shocking reveal he's quitting the show, there's no reason to believe there won't be a 16.
Besides, even though for the past few seasons, for some reason, FOX has been low key with Family Guy's renewals, I have no doubt once the decision has been reached between Seth & FOX to end the series it would be announced prior to its airing that 'X' season is FG's final season, it would be quite surprising to have it quietly fade away.

Man I hope so. Season 14 and 15 are really good seasons.