A few days ago, for example, someone (Brendan Moody ?)said that maybe only 50,000 people would care about Paul McGann's Eighth Doctor. Fair enough point but surely he's at least as important as Hurt's Doctor? Those who don't already know him can come to be interested in him at least as easily as they can become interested in Hurt's Doctor.
The difference is that John Hurt is, to use your words, an acting titan, and Paul McGann (although he's very good as the Doctor, particularly in the better Big Finish audios) is not. I'm not sure I buy this "secret Doctor" notion, except possibly as a fake-out, but from a promotional perspective, "One of Britain's most famous actors plays a past Doctor" is a bigger deal than "the 'I' from Withnail and I
plays a past Doctor."
Anyway, whatever role Hurt is playing, I doubt it's at the expense of a classic Doctor. It'll turn out that what Hurt's character is doing couldn't, for plot reasons, have been done by McGann or Davison or whoever.
On a related note, I think people need to let go of the idea that this being marketed as the 50th anniversary special means it's more likely or more obligated to integrate the classic series in some way. Unfortunately, "50th anniversary" is more a marketing opportunity, a brand, than a content description. It's an excuse to sell more merchandise and do one more "event" episode of the current version of the series, with the same indifference to 1963-1996 as ever. I wish it could be different, but the popular perception of one continuous series that allowed big continuity fests in 1973, 1983, and 1993 no longer exists.