tl/dr: The purpose of Genesis 1-9 is quite clear: to teach us where we
came from, why our world is a mess, and why we need a savior. If you missed that part I
suggest you go back and read it again :)
If you believe that the Earth is a sphere, then why does the Bible refer repeatedly to the Earth having four corners? Surely you don't believe that all dozen or so references to corners of the Earth are metaphorical? If you do, then where do you stop? Is the new testament metaphorical? Is Jesus' death and resurrection metaphorical??? Is God a liar and our faith powerless????
The Central Question
It all boils down to one question: when God wrote the book of Genesis, what was His goal? Did He intend to provide a precise, blow-by-blow description of exactly how He made it, or did He intend to help us understand our own origins and - more importantly - the fall (or maybe both)?
The main goal of the creation story is indisputable. The key takeaways from Genesis are the fact that God created a beautiful universe out of nothing through His spoken word, and that mankind rejected His perfect will and made a miserable mess of the place. These are, without question, the most important things we learn at the beginning of the Bible.
What else though? Did God also intend to provide a precise, blow-by-blow description of exactly how He brought our world into being? Quite frankly, I don't think there is anything in the Bible that lets us answer that with any certainty. I think that is much less critical than the main goal - helping us understand our origins. The Bible certainly is filled with allegory, parables, and metaphors. The story of creation can be any of those, and it need not cast anymore doubt on the Bible than Jesus did by telling parables. If you think that YECism is the only way to understand Genesis or else the whole Bible falls apart, then I suggest that you need to have a little more faith :)
Sure, many Biblical scholars throughout history believed that the creation story was meant to be a literal description of how creation happened. There were also many who didn't believe that. As always, the most important thing is to give grace on topics that are unclear, and for me this is clearly a topic on which the Bible is unclear.
Sad But True
Sadly, during my college years I spoke to many college students who had grown up in YEC families. They went to college, took some science classes, decided that everything they had been taught was a lie, and walked away from their faith. Of course, who knows whether or not YECism was really what drove them away from their faith, but I ask you: why give someone the excuse? I'm not saying that everyone should abandon YECism just because it doesn't fit in well with modern sensibilities. However, I would invite you to very seriously answer this question for me:
Do you think that YECism is a topic that is worth shipwrecking people's faith on?
If your answer to that question is YES!! then I would once again encourage you to have a little more faith.
Let's get crazy
Ironically, I think that some of the fervor for YECism is a result of drinking too deeply from the very same kool-aid that YECs accuse modern day scientists of drinking from. Regardless of how old it is, YEC is very much in line with the spirit of this age (and not in a good way).
"Scientists" like to understand everything analytically, have a precise understanding of the rules, have a detailed and accurate history book to review as needed. We expect and strive to have a perfect and clear understanding of everything around us and everything that came before us. We've used that mindset to do an amazing job of understanding physics, history, and the universe. We've used that knowledge to transform the world. I think that modern day YECs take that exact same mindset and incorrectly apply it to the Bible.
Just because you can read the creation story like a history book doesn't mean that God was writing a history book in the modern fashion. Frankly, I think it takes quite a lot of modern day hubris to look at the creation story in Genesis and say, "Ah yes, clearly God was writing this just like I would have if I had been there and taking notes, so I will interpret it that way". In essence though, this is exactly what YECism both believes and demands.
Let's get more crazy
So what does the creation story tell us about how the universe is made? I'm not sure, which is why I'm generally happy to let people believe what they want. Having the wrong belief in this area will not stop someone from believing in Jesus, despite the extreme importance that the Christian church seems to give this question. As a result, I've intentionally decided to not have an opinion on this topic. There are more important things to discuss.
However, I suspect that it is indeed factual, not a metaphor or allegory, and I also believe that the universe and Earth are billions of years old. I expect that the apparent disagreement between these two facts is just that - apparent and a result of our limited view and limited understanding of God's power. When we get to heaven, assuming we still care, I expect we'll get the full story from God and will be able to honestly say, "Yup, the book of Genesis got it perfect" while also saying, "Yup, the Earth really was quite old".
An Illustrative Example
Countless people in Jesus' day missed Him because they knew that the Bible promised the Messiah would come as a conquering king. When Jesus didn't meet their expectations, they refused to follow Him. The crazy part is that they weren't even really wrong. We know that Jesus will return again as that conquering King promised throughout the Bible. The Bible was true, it's prophecies were true, and Jesus is the Christ. The only thing wrong was people's interpretation of things. In exactly the same way, the Earth can be old and the Bible can be true. It's as simple as that.