I'll believe the "last raid" comment when I don't see Ruby Sanctum. Deathwing being the main objective of the expansion doesn't guarentee he will be the last content patch of the expansion as neither Illidan or Lich King were last in their respective expansions.
I will quote mmo-champion on this one.
"it very likely to be the last patch"
Until I see a release date for an expansion though, "very likely last" and "is last" are not one in the same. All comments about it being last or possably last are by the community, not from Blizzard themselves.