Livre d'Or

Yes, I think you're right, the one good idea of an allegorical retelling of the Passion isn't enough to sustain the whole seven-book series. There's still some very good portal fantasy round the edges, but the bits where it's squeezed to fit the Christian allegory framework don't work very well. Regarding the pastede-on Christian bits as tedious advert breaks makes a lot of sense.

It seems to me the best audience for the Narnia books are precocious kids, advanced enough to read a fairly complex story but young enough not to question the underlying theology / allegory. I think that's why the characters in Lev Grossman's The magicians are so obsessed with their thinly-disguised equivalent of Narnia; I found that a very realistic portrayal, though I think Grossman's critique of Narnia doesn't work well.