That is a very laudable attitude to take. I appreciate your stance on the matter.
As to the actual subject of discussion:
I think you do my argument a disservice in calling it an "assumption" (not taking it personally, just pointing it out). If darkvision cannot penetrate this darkness, then obviously mundane sight cannot either, as the latter is in every way inferior to the former. It is a matter of common sense. Not to imply you don't possess common sense, just that you don't seem to view its application as necessary here, because the rules already provide complete information in your eyes. But as I say it, darkvision is not a completely separate sense like blindsight or tremorsense. You cannot logically make the argument that mundane sight can see something where darkvision cannot, solely because WotC failed to provide wording for it. Nor do I think you can disregard wording that *is* there because of it.
I think what we have here is a conflict of RAW vs RAI. From your explanations so far, I can tell that we lean towards different ends of the spectrum. Filling in the gaps that incomplete wording leaves is not the same as actively changing how a rule works (as in: a houserule).
The comment regarding illumination inside the spell was meant to briefly touch upon your arguments regarding the rules reflecting how sight operates in reality. I already dread the side debate on the exact workings of the eye, but to my limited understanding if light does not illuminate anything (therefore is not visible to the eye), it will cause no reaction in the retinas and you therefore cannot see.
We run into a similar issue here. You say the rules do not explicitly say something and therefore it cannot be the case. Whereas I say we have the necessary information to fill in the gaps. If the eye cannot perceive the reflected light, then it only logically follows that it cannot see anything that light was reflected off of. To me, that is not an assumption, it's like solving for a variable in a mathematical equation.
Also, I want to point out when I say something is "effectively" blinded/opaque, I simply want to express there is no functional mechanical difference, despite the term itself not being used in the wording I am referring to.
Somewhat ironically (after everything I said about RAW vs RAI before), we come to the part where you bring up the design intent of Darkness. And yes, straight up, the spell is bad. It is a glorified fog cloud, with a smaller radius, different conditions for its removal and the option to cast it on an object and carry it around. It does not work for Drow as it did in older editions anymore, nor does its then tried-and-true combination with faerie fire. The combination with senses that can perceive through it (such as the devil's sight example you brought up) is its main valid use in a combat scenario. You can also block line-of-sight with it, which can be useful against enemy casters or neutralize existing advantage/disadvantage on the battlefield, but apart from that...
As for how WotC arrived at its current iteration and why, who knows? But I think we can both agree that it is not the only spell that is not worth taking due to lackluster design (unless we are talking certain niche applications).
Finally, I think this debate about rule wording and interpretation might detract a little from the actual point of the thread - or at the very least clutter it. If you like, we can continue this exchange over PM. If you think our viewpoints are too fundamentally incompatible to continue, we can also drop it here.
In any case, I very much appreciate the civility so far, despite our disagreements.
