It should not be difficult to create a modern, elegant and responsive theme.
Creating a modern, elegant, and responsive theme independent of Centova Cast is not difficult at all. Integrating it with a project of this size, though? That's another story.
Is it really so complicated?
Actually yes, it's massively complicated and a huge undertaking.
Creating a nice RESTful architecture to sit behind it would require further major changes under-the-hood.
So yes, it's certainly possible. It's not something we're going to drop all other development for, however.
That said, we ARE working on modernizing Centova Cast, but the first step is modernizing it under-the-hood. We currently have an active development branch where we're refactoring Centova Cast to adhere to PSR standards (remember that Centova Cast was conceived back in 2005, when PHP4 was basically it) which, in the process, will dramatically improve ongoing maintainability and overall performance. That's all under-the-hood, but once that's done we'll be overhauling a number of user-facing components, including the UI. I won't go into further detail on that at this point, but suffice to say we've got a long list of plans to keep Centova Cast relevant moving forward.