", or "No shit! 
".I agree that the "I am your father" is ruined for the audience, but I think it still works on a dramatic level to have the surprise be just for Luke. We see that Anakin is Vader in the prequels, then we see Ben flat out lying to Luke about Vader killing Anakin, so after Yoda and Obi-Wan beg Luke not to leave before he is trained because he's not ready and he leaves anyway, we get to that part where Vader tells him the truth and the audience then wonders... Crap, how is Luke going to take being lied to by Obi-Wan? Is he going to be able to still trust his teachings? Is he going to say screw Obi-Wan, he's no better than Vader as Anakin said about Palpatine vs the Jedi?
We don't find out until the next movie, which starts out with Luke dressed up like Evil Anakin in black robes, force choking gammorean guards and telling Jabba not to underestimate his power.
To me this whole dynamic ratchets up the tension quite a bit with the way it all flows now.
Again nothing against the landmark surprise of "I am your father", no doubt that is one of the biggest surprises in cinema history.
But is is a very fleeting surprise.
Now, to keep that surprise, you have to show a first time viewer the classic trilogy first. The problem with that is you lose most of the surprises in the prequels. You know going in that Palpatine is Sidious, you know going in that the Jedi are going to be made all but extinct, you know going in that only Yoda and Ben survive, you know going in that Anakin does not do the right thing in the end, you know going in that Padme won't survive. You know going in that the Republic will become the Emprie. The list goes on and on.
All for one surprise that only works the first time you see it.