To me it is unmistakable that the 'days' in Genesis 1 are used as a polemic to counter pagan view of unfinished creation continually occurring each day, as well as a literary and didactic device, as well as a beautiful structural device to further emphasize the order God brings in creation, as well as temple liturgy where creation is presented as a 'cosmic temple'. Again, I don't think this because I'm looking for an 'out' with long ages of science. I truly see this evidenced in the text itself.
HOWEVER, even though I'm convinced of the above that still doesn't rule out the possibility that AN ADDITIONAL purpose is that the six days are ALSO meant to be taken as a statement that God really did create everything in six days. I think the strongest arguments for this are Exodus 20 (although the emphasis is not on the days, but the establishment of the Sabbath) and the fact that unlike Christianity later Judaism consistently interpreted Genesis 1 this way.
*So believe it or not, and despite all appearances to the contrary, I'm still open to the possibility that Genesis 1 teaches this too.
I think that's very wise.
I try to separate science and scripture when looking at this question to see what they independently say:
Based on what I know on the science side, I see the conclusions of modern science as being pretty decisive when it comes the universe and earth truly being old and not young (*decisive like how we know the earth orbits the sun and not the other way around type of decisive). So I don't see science changing on that. But even for sake of argument if radiometric dating is wrong, and for argument sake EVEN IF the earth and universe are NOT billions or even millions of years old, 6,000 years is simply not enough time for the geological record to be produced and for the geological changes that have occurred on earth to happen--like continental drift, for example. The earth has to be much, much, much older than 6,000 years (*the YEC catastrophic plate tectonics theory of accelerated continental drift and radioactive decay during the flood--while potentially promising at first--doesn't work, but would produce enough heat to boil away the oceans and fry life).
But on the scripture side it looks like we've found another point of agreement! The Bible doesn't clearly say either way. As explained above in the post you quoted, I also see some difficulties with interpreting the six-day creation as meant to be a factual statement of the actual time it took for creation, since to me it is clear that Genesis 1 is a direct theological confrontation/polemic against Egyptian pagan theology that occurs on a theological level. For example, if it's true that Genesis 1 is a repudiation of Egyptian creation accounts, then Genesis 1 is merely following the same order of events as the Egyptian accounts (supernatural light created first from formless/void, primeval waters, then separation of the waters by a firmament expanse, then appearance of land, etc.)---AND the 'real' purpose of the six-days is then to counter the daily, ongoing self-generation of the gods (like the sun god, being reborn each day), cyclically without end, by presenting creative acts as decisive acts of God that did not keep going on and on and on in self-generation, but came to an end with the ceasing of creative action on day 7. In this way the seven-days would counter the Egyptian belief of creation 'not finished' but still occurring each and every day, with the finality of a finished creation. In addition, Genesis 1 refutes the theology only, but does not correct the erroneous, pre-scientific view of earth in those accounts (e.g., the light of daytime created on day 1 because they didn't know the sun was the source of that light, the 'cosmic waters/ocean' ('heavenly ocean') put above the firmament with the sun, moon, stars beneath that, etc.). But this is only a problem if we insist on concordism; if we insist that Genesis 1 and science must ultimately harmonize. But if it's a theological rebuttal, then with the exception of a rejection of metaphysical naturalism, Genesis 1 and science are apples and oranges that don't speak to each other and have little to do with each.
*While I know you disagree, I hope you can at least still believe me when I say that I'm honestly not trying to interpret Genesis 1 this way for the purpose of trying to solve the apparent conflict with science. I truly do compartmentalize the two and look at science and scripture separately and independently of one another: i.e., What science says, and what scripture says independent of science, so that I'm not tempted to twist scripture to fit science or science to fit scripture. And so for what it's worth, the theological polemic against Egyptian paganism that I see as the original context and intended meaning of Genesis 1 is truly based on my sincere attempt to understand what Genesis 1 would have meant to its original, intended audience. That is, I could be sincerely wrong, but it's still an honest attempt to know the *right reading/understanding* of Genesis 1, with no ulterior motive to solve any apparent conflict with science about old ages. I truly don't let modern science affect my reading in that way, and strictly seek to understand the true, intended meaning...as we both are.