Related: How was Harry supposed to defeat Voldemort in Dumbledore's original plan? by @Arthur
Related: What were Dumbledore's actual intentions for the Elder Wand? by @NominSim
This question is not -- is not -- asking what Dumbledore's plans for the Elder Wand are or who was supposed to get the Elder Wand (We already know Dumbledore originally wanted the wand to go to Snape; I don't at all doubt this was Dumbledore's intention.). I'm asking why, after such detailed and calculated planning, Dumbledore so abruptly (at least it seems so) allowed his intricate plan to be derailed by Draco Malfoy?
Harry hurried over to the door leading to the spiral staircase, but his hand had only just closed upon the iron ring of the door when he heard running footsteps on the other side. He looked round at Dumbledore, who gestured to him to retreat. Harry backed away, drawing his wand as he did so.
The door burst open and somebody erupted through it and shouted: ‘Expelliarmus!’
Harry’s body became instantly rigid and immobile, and he felt himself fall back against the Tower wall, propped like an unsteady statue, unable to move or speak. He could not understand how it had happened – Expelliarmus was not a Freezing Charm –
Then, by the light of the Mark, he saw Dumbledore’s wand flying in an arc over the edge of the ramparts and understood ... Dumbledore had wordlessly immobilised Harry, and the second he had taken to perform the spell had cost him the chance of defending himself.
Half-Blood Prince - pages 545-546 - Bloomsbury - chapter twenty-seven, The Lightning Struck Tower
I have a bunch of ideas regarding why Dumbledore sacrificed The Grand Plan in order to ensure Harry's safety, but certainly I haven't thought of everything. So I am asking the site:
Why did Dumbledore allow Draco to disarm him?
I'm looking for an answer grounded in canon (Harry Potter books or J.K. Rowling interviews/quotes or Pottermore); subjective answers in the spirit of canon are also very welcome. I am not looking for an answer from the HP Wikia.