I agree. I was hoping to see a fresh cast, preferably French.
I won't exactly say Sierra is too old for the part. But to me she always seems to have a poise and maturity that didn't quite fit. I first saw her in LND, and that was fine because it was a 10-years-older Christine. But when she was the (original) Christine in the 25th Anniversary show in 2011, I felt she lacked the girlish quality I look for. Leroux doesn't exactly spell out Christine's age, but I think she ought to be around 20-22, with a childlike naivety that I don't find in Sierra.
And it must be annoying for any up-and-coming French sopranos who'd have loved a chance at the role. But maybe Sierra is to be the big-name "bankable" star to pull in audiences, and local talent will get their chance in the other parts.
More info on Sierra's casting here: