Nah, they won't lose. With the new Vios pending, they can easily claim it is a new generation with many new exciting features such as

1. 2 more airbags - that's 2 more from the current model
2. VSC - There's an extra safety feature! Only problem would be they having a harder time explaining why such a feature is omitted from the new Camry.
3. If they do use a new engine, it would probably be the new 1NR-FE engine. The 1NZ-FE has been a solid performer for many years, I can only imagine the 1NR or 2NR-FE to be even better. "New" engine also proven reliability and spots dual VVT-i, can charge premium somemore!

I suspect the Prius C not doing very well largely due to the car being a hybrid. But it's not exactly a poor seller either, on average I see at least 2 or 3 Prius C on the road everyday.

As long as the car is as good or slightly better than the current Honda City, they will sell.