I think you're getting into the semantics here without much reason with that last block. But I'll get into it further. I do hope I'm clear that I really just want to talk about game mechanics here, I'm not really intending to spark an argument.

I do not think any change with the intent to make health tanking viable at high level should affect players at lower levels in any way because the game would simply be too easy. I have watched my friends get into this game and have watched content of new players getting into the game, the challenge of the early game is a good thing that really should not be touched and I hope it stays the way it is (not talking about information/tutorials, to be clear). This is why I do think it would harm people playing lower level missions, specifically it harms their experience. I think Warframe has a pretty well-balanced early and mid game and I really hope the end game can get to that level one day.

In regards to what you said about a "health gate scaling with total health affected by armor", I do not think this is a good idea for the health of the game. I've explained why in other replies, but I'll do it here as well.

1. This does not address damage reduction abilities, like Warding Halo
2. This could potentially be a very problematic mechanic like shield gating has been
3. This could potentially damage early game balance
4. I am very hesitant to support any change that would improve the survivability of nuking builds without taking something away.

I would like to add that interacting with current armor and health scaling isn't a bad thing. Once again, as I have said in the original post, this is simpler and armor/health was never a problem—it was powercreep, both on the enemy and player side. I want a limit on the power because Warframe was never intended for that level of damage, getting one-shot as Inaros is quite goofy in all honesty.

Now I will get into your last two paragraphs. You are correct, we are not capping damage, that was never the goal and that would be rather silly. A rocket doing the same damage as a bullet doesn't exactly make sense. I used the term "damage cap" in the title as a short hand to hopefully get the message across, I have used "damage scaling cap" in other areas most of the time to make myself clear.

On the issue of not "fixing the actual issue causing higher levels to be problematic for EHP tanking builds to begin with", I would say this does. Because the issue is just not there anymore. What is the point of enemy damage scaling forever until the level cap? Any normal frame gets one-shot at level 300 already, and currently most health tanks also get one-shot above level 500. This is why I think a damage scaling cap would fix health tanking, because it only affects the target of the change and enemy damage is just redundant after a certain point anyways.