Spoiler In oblivion you save the word by helping martin, what if you could join the opposing side and give them the amulet of kings and destroy the world. That would make it good for both evil and good characters
Tamriel would be overrun, Mehrunes Dagon would take control and everyone that he decided to spare would become his eternal slaves. It may suprise you, but I doubt he planned to keep his promises of immortality and power once he had taken control.
There's also the question of continuity, TES is a lot more lore focused than other games; so it's difficult for Bethesda to say "This ending is the real one". Last time that happened was Daggerfall, and we got the warp in the west. I'm not this knowledgeable on lore, but I'm pretty sure that was caused by Akatosh. Seeing as how we're fighting him (or his Nordic equivelent at least), I doubt that's going to happen this time.
Of course, I do still like the idea of alternate endings / stories, up to a point; I think Morrowind is a good example of this, It gave an alternate storyline to follow, which had questionable moral decisions to make, and it the long run, it doesn't have a major impact on continuity
Spoiler Vivec dies eventually anyway
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In fact, I have a copy of the OXM right here, and regarding radient story it says "By the endyour version of the main quest could have played out very differently from that of your friends." - this suggests to me that beth's radient story won't just be limited to side quests, so it's fair to assume that there will be certain aspects of the MQ that will differ between characters, depending on your play style.