Vampires want to block out the sun, and you have to stop them. I would like there to be the ability to join them and block it but if you think about it, that would ruin the whole game and be an automatic endgame.
So an Elder Scroll can close the sun? And if the sun were closed, would magic be unusable?
If you block out the sun, everything in Skyrim would die, so saving the world from Alduin was pointless as everything would die anyway. It would also destroy the visuals of Skyrim that Bethesda spent so long creating.
Has anyone considered that
'blocking out the sun' is a special ability or once-a-day power?
An ability to cause an eclipse effect allowing the character to walk about during daylight hours?
Naysayers: "If I can block out the sun then all of Nirn will die and the game will end! Why'd we stop Alduin?! OMG Beth is soooo stupid!" 
Seriously? Is this how some think of sun blockage rather than, perhaps, a once-a-day power?
No matter the subject, it seems some are never happy unless they're unhappy.
Odd, that.