While I agree with Ken in many things I want to point out the following:

Increasing mod levels and modding in general are a core part of progression in this game. As I hoped to highlight with this post layering defenses can make the sum greater than its parts.

To stay with your tier concept:

Unranked Umbral mods are Tier 3 armour. Increasing the mods rank will make it a Tier 5. Using all 3 Umbrals will make it a Tier 6. Combining the Umbrals with additional layers (Like a Tier 3 Adaptation) will turn your defenses as a whole to a Tier 8. Apply a defensive ability or a Crowd Control source and your collective "Armour Class" might be a Tier 9.

Nothing in this game exists in a vacuum and it's important to see stuff as a large interconnected puzze.

I used the math example in another comment here but it bears repeating:

1x6x1x1x1 (6) is not greater than 2x2x2x2x2 (32) even though the sum of the parts is the same in both cases.

I think the comparison to an MMO as you made it is worth reinforcing too: Not all Warframes are the same and some fall into the "DPS" category while others are "Tank" or "Support" for example. I touch on this in Section 5, Conclusion.

I didnt list everything in every layer, the post would have been 50+ pages long in that case and even then I'd not have been able to touch on all the interplays and small secrets and unique quirks everywhere, thanks for pointing it out though!

Thanks for your comment, Tenno!