This is a non I have been considering for quite a while now.
S -> S-
OR
S- -> S
While I still do think Landorus is an amazing pokemon currently, I don't feel it's a big cut above the other mons in S- to justify itself being a tier higher. With trends in Rillaboom fats, Landorus finds itself to have more competition as the team's SpDef Ground Type, from the likes of Hippowdon and Garchomp. Landorus not only has it's Earthquake weakened, but it can't make use of the terrain, so it won't gain increased survivability. Another archetype with an uptick in usage is Sand. Excadrill or even Hippowdon have been the main ground type of choice on these teams. Landorus also can get overwhelmed by Choice Banded moves from Excadrill when running a SpDef set. Landorus if it wants to be a hard wall to Excadrill must invest in physical defense, which would leave it vulnerable to other threats like Dragapult, Zapdos, Heatran and Blacephalon. Lastly, Landorus is extremely vulnerable to Knock Off, and losing its leftovers prevents it from being able to check Tapu Koko or Zeraora late game. Other ground types like Hippowdon and Garchomp can make up for this, by having Grassy Terrain recovery on Rillaboom teams, or in Hippowdon's case, instant recovery. Now while all these points may make it sound like I'm saying Landorus is bad, that is not the case. Landorus is still the most splashable out of any of these and provides Defog and U-Turn, which let's it support it's teammates even further. The flying typing also makes it even more annoying for the opposing Ground Types, as well as having a U-Turn resistance. There is much more good I can say about Landorus, but it's pretty self explanatory. I still think Landorus-Therian is incredible in this metagame, but I don't see it being a full tier above the 4 in S-
S -> S-
OR
S- -> S
While I still do think Landorus is an amazing pokemon currently, I don't feel it's a big cut above the other mons in S- to justify itself being a tier higher. With trends in Rillaboom fats, Landorus finds itself to have more competition as the team's SpDef Ground Type, from the likes of Hippowdon and Garchomp. Landorus not only has it's Earthquake weakened, but it can't make use of the terrain, so it won't gain increased survivability. Another archetype with an uptick in usage is Sand. Excadrill or even Hippowdon have been the main ground type of choice on these teams. Landorus also can get overwhelmed by Choice Banded moves from Excadrill when running a SpDef set. Landorus if it wants to be a hard wall to Excadrill must invest in physical defense, which would leave it vulnerable to other threats like Dragapult, Zapdos, Heatran and Blacephalon. Lastly, Landorus is extremely vulnerable to Knock Off, and losing its leftovers prevents it from being able to check Tapu Koko or Zeraora late game. Other ground types like Hippowdon and Garchomp can make up for this, by having Grassy Terrain recovery on Rillaboom teams, or in Hippowdon's case, instant recovery. Now while all these points may make it sound like I'm saying Landorus is bad, that is not the case. Landorus is still the most splashable out of any of these and provides Defog and U-Turn, which let's it support it's teammates even further. The flying typing also makes it even more annoying for the opposing Ground Types, as well as having a U-Turn resistance. There is much more good I can say about Landorus, but it's pretty self explanatory. I still think Landorus-Therian is incredible in this metagame, but I don't see it being a full tier above the 4 in S-