People keep saying that Gravis is the replacement for Terminator Armour, but I just don't see it. It doesn't have a better save, no invulnerable, no teleportation... none of the hallmarks of Terminator armour.
I also don't agree that First Born Marines are going to be made Legends. While it is true that GW heavily favours new players, telling their community that the collections they have been building for decades are now no longer useable in standard play would be the worst PR decision in their history.
Space Marines aren't like Squats, where they can quietly sweep them under the rug because only a handful of people play them. They were the corner stone of their business for decades. People have built entire Chapters, and then some.
Bladeguard replaced assault terminators
Except they didn't. Bladeguard are a strong combat unit, but they don't have the same strengths that Assault Terminators have. They are actually the middle ground of what was missing.
Terminators with Lightning Claws excel at clearing one wound models, Bladeguard excel at clearing 2 and 4 wound models, and Thunder Hammer Terminators excel at killing everything else and
being harder to wound.
... though I still think GW are trying to dump terminators...
As a Dark Angel player, I think you're going to see a resurgence in Terminators when the supplement comes out. Deathwing Terminators are already great as it is.