People use the signaling argument all the time and I don't think it has ever meant shit. Where can we find examples where signalling has had an effect on FAs signing with a particular team? People said the same thing about Jayson Werth.
Happened with the Phillies when they went out and signed Thome after CBP opened. All the signing did(and its the same with Seattle's situation) was tell the fans they were willing to spend the money. His signing ended up meaning nothing because the team didn't take off until Utley and Howard came along a few years later and became the faces of the franchise along with Jimmy.
So yea, I typed that to say I agree with you. The big FA (that's meant to signal a franchise turning the corner) doesn't mean a thing unless there is a core in-place(or soon to be) around him, preferably through the farm. Its the only reason why the Phillies and Natinals took off... And I suspect the same will have to happen for Seattle if they wanna make this Cano signing matter.