I think the points people were making are that it's more thematically appropriate for the Divine to take on Damian and that he's the only one powerful enough to realistically stand a chance. And in theory, Damian already had reason to fight the dragon knight as soon as he started destroying his fortresses and killing his generals.
In any case, I fail to see how any of this makes the game a ripoff. Plenty of games have bad endings - the final areas are often the most rushed part of the game due to the developers running out of time - and some barely even have stories at all (e.g., roguelikes). What makes them worthwhile is the fun you have playing them, of which the last battle is only a minor part.
Regarding your other point, trailers often contain content that aren't in the game, since they come months before the game has actually been finalized. It would be one thing if they marketed the game as having features it didn't (Bethesda and Lionhead have been pretty bad about this in the past), but showing a scene that was later cut hardly qualifies.
