My main issue with Miller's idea other than Superman committing suicide at the end is the scope of it in 3 films. There is no way superman's life will fit in 3 movies. Lets map it out.
Before he even becomes Superman we have to redo the destruction of Krypton. I think it could be done so much better these days but is pretty unnecessary for a regular superman story. If they want a life story though its best to start here. They need to introduce his parents and his life in Smallville, Lana Lang and Pete Ross. No doubt the entire Superboy/Legion part of his life will be removed. He graduated University and heads for metropolis where he works hard and gradually moves up the ladder to the reporter we know. If we want to do 'superman's life story' you could do the first movie on superboy alone but Miller won't. The best villain for an origin story is Brainiac as he works on a cosmic level.
More likely that would be done in an hour and a half in the rush to get to Lois, the Planet and that whole area where a regular Superman movie would begin. Now we get into the main part of the trilogy the villain no doubt would be Lex Luthor. The whole other part of his life, the justice league/DCU part would most likely be completely removed. Get the feeling the 'superman life story' is missing a HUGE chunk by now? I do. Then we get to Supergirl and Clark realising he is not alone. This could probably be worked into the movie and Lex's involvement in a Superman/Batman:Public Enemies type way.
Now what? We have to get from Clark in his peak to the end of earth. Thats a while. No doubt it would be a different take on the myth. No Lois. No Smallville. No Daily Planet. A true opportunity to do a cosmic story but not exactly a life story. We would need vast backstory about what happened between the two movies none of which would go into detail. Perhaps this could work if they have a villain that chases him and eludes him through time.
Miller's canvas is way too large and even if he could tell superman's life effectively given the time span there is no doubt key moments would be missed.