That way we lose the subjectivity (as much as we can) regards the 'this game does everything so is the best in graphics.'
Not really. The basic difficulty in these discussions isn't that people disagree about what's objectively going on; that aspect of the discussion is usually perfectly healthy. The difficulty is that different people weigh things differently. What the graphics are doing at a given moment is objectively quantifiable, but broad "graphical quality" is a subjective determination. The output of a rubric that weighs objective factors into a few "quality" numbers is essentially an opinion, and fundamentally not much different from asking a person what they think. All you accomplish with a rubric is that people will criticize the rubric, which isn't really a change from what happens right now.
And what about constructing that rubric? There are so many qualitative variations, and so many things whose significance depends on other things. Even something as basic as size and expressiveness of dynamic lights is an insanely complex
realm to judge. Most lights in Halo 1 are enormous and can cast (white) specular; Halo Reach can handle far more lights and supports specular color, and the specular is more expressive; but most of these lights are smaller and many diffuse-only, so in some ways Halo 1 often sprawls more interesting dynamic light across the screen. How do we rate these two very different cases against each other in a way that is both useful and can be evaluated in a purely objective manner? If we can't evaluate it in an objective manner, then the rubric is worthless, since different people will come up with different numbers, which violates the whole purpose of having this "objective" rubric. And if we simply cast aside these kinds of complexities and leave them out of the rubric, the rubric is worthless, because there are hugely significant aspects of a game's graphics that are being totally ignored.
And what do we do with the rubric? Humor the conversation with it as a starting point? If someone makes a dissenting opinion, the rubric wouldn't have any kind of special (intellectually honest) leverage against that judgement, since it's ultimately an opinion as well.
The reasonable thing is for people to stop panicking over whether a game's graphics are the best. Comparative discussion is great, and it's entirely possible to have a healthy discussion where people explain why something is their favorite. The issue is that people turn it into a battleground where the goal is to
win. Creating a rubric which can be evaluated from objective data won't change that.