Her words might be echoing N'zoth's however and may have for some time. 'No king rules forever' was a Terenas ghost- and a Yogg Saron-quote at the same time too.
I still don't think that she's outright controlled by an Old God, but I do think she follows the footsteps of several others who thought they were doing their own thing and realized too late that it was what 'the whispers' wanted all along. Since in Cata the Forsaken in Eastern Plaguelands told me he left the Forsaken, because Sylvanas has lately started acting too strangely for his liking I was quite convinced she was being manipulated into furthering the Old Gods' goals.
I am also convinced that this is why the Loa wanted her to step out of the shadows, so she can either be turned around or be stopped before it's really too late, because people would notice when the Warchief goes nuts. And maybe it even sped her 'descent' up a bit, because she had to adjust her plans and get rid of people who would stand against her and try to stop her, if she went too far in their eyes. She has to seize complete control of the Horde, if she wants to bring her plans to fruition. And the more of the Horde she can push to go along with her plans, the less of a chance anyone has that there will ever be peace again, even for a second. So they'd have no more chance to betray her and turn on her, because then they'd have nowhere left to go.
It might all also still change completely and nothing of this will actually happen. Or it may really be a false flag kind of thing done by N'zoth.
Anyway, interested to see where this all leads and how it plays out.