Looking back to the past can give us some idea of what we might expect to happen in the future if circumstances remain at least somewhat constant. Sony still has huge mindshare in the industry, X360 still has trouble in Japan (and maybe Europe), Sony still has many titles that sell a lot of copies, so anyone who is saying that PS3 might retain their lead is simply looking at these facts.
Of course, looking back at the past can cause us to be blind to the present/future and thus we shouldn't focus solely on the past but also the present and future. The present is that Sony hasn't even released their console so we can't even see how well the console is received. Sony is currently having some production problems, but how long will those last? No one knows. MS certainly has gotten quite a few good exclusives, but will the PS3 draw back more developers and exclusives if the console ends up selling more than the MS console again? None of these things can be known.
So basically, my whole point here is that people are just making their best guesses about what will happen in the future, and at this point all they can be are guesses. Ever heard of people arguing about how many stars are in the sky, if there's life on other planets, etc? No one knows for sure, but people still like to debate these topics and others. Nothing is wrong with that as long as people remember that until something can be proven as a fact, then every opinion is valid... although some are certainly more plausible!
