They were destined to be Lucifer and Michael's vessels and act out the battle that would wipe out most of the earth's population.
That didn't happen. Dean plainly didn't say yes, and even more importantly, Sam said yes and flipped Lucifer the bird, because the power of love did win the day.
It wasn't destined for the earth to come out of it mostly unscathed. But it did, because Sam was that strong, and because Dean wouldn't let him be.
It absolutely mattered what they did, and who they were. Look at poor Adam! I hope that whatever force (God, probably) got Castiel and Sam out of their jam put him back in heaven and maybe he finds his mother. But he didn't have what Sam and Dean had, and he wasn't able to stand up to the powerful forces arrayed against humanity.
I think the Lisa thing is wrong, but I totally get why both of them think it's right, so I don't mind Sam extracting the promise and Dean following through. I will be disappointed if Show thinks it's right, though.
eta:
And Sam's in hell and Dean's in hell, and really, what the hell?
Sam's in Hell? Who do you think is under the streetlight, then? I thought JP did a good job of differentiating between Lucifer and Sam, and I thought I saw Sam. A different Sam, but Sam there.
Dean's seeking an end to his misery, and in the wrong place. Time enough to get that resolved. It didn't need a bow on it at the end of S5. I wouldn't have expected it--it doesn't jibe with the season enders that have preceded it.