Well, there's only two more tenets to come, but from what we know of Ventari, and the unlikelihood that he predicted that the sylvari would be born, I doubt there's anything in there that enshrines the Firstborn as an upperclass.
I'm also not convinced they are a permanent upperclass. It's been mentioned that Caithe regrets her words, and in the context her intent could simply have been to point out that compared to the seven-year-gap between the first and second generation, there's nothing special about being the first of the second generation. Beyond that there may be no entrenched difference - the sylvari could well be a meritocracy, it's just that the Firstborn have had that much longer to demonstrate their merit.
The problem with the argument that the Firstborn are there on merit, is shown by the situation with the krait. As mentioned above, there is a logical and rational reason to want to kill them; one cannot question the utility of it. The problem is that it is an ethical and moral decision, and the Firstborn that was with Cadeyrn did not offer elaboration, merely recitation.
Again, the problem becomes in that (if I remember correctly), the Pale Tree shows an obvious preference to the Firstborne. For example, only speaking to the Firstborn during the decision on whether to go to war on the Asura or not. And in truth, though Caithe may regret her words, the regret could (and it seems more likely to) come from being overly blunt. In essence, an excess of honesty. Caithe's words encapsulate the split in Sylvari society between Firstborn and 'everyone else'.
The merit argument also fails to hold weight, because as time progresses, it would seem rational that we see other Sylvari rise to positions of power, or as Luminaries. After all, the Firstborn only have 7 years of relative isolation, which as time goes on will become miniscule. For instance, at the time of GW2's launch, the Sylvari have been around for 23 years. Roughly 1/3rd longer than other Sylvari, it's true... but a short amount of time, considering how long Sylvari society will be around. When the Sylvari are, say, 50 years old, will 7 years really matter? Especially 7 years of learning things that every Sylvari will learn that came after them? The fact that Sylvari have 'inborn knowledge' rather diminishes the advantage that the Firstborn have, since all the knowledge they get goes back to the Tree.
In the end, I'm not here to make the NC's case for them; the point is, a case can
be made against the Tablet/Tablet practitioners, the problem with the NC is that they define themselves by their opposition to the Tablet, and that definition inherently causes them to fail at being 'natural Sylvari'. They are limiting themselves based on 'it's in the tablet, thus we should not do that', which is still giving the Tablet an influence over Sylvari culture.