1. Compared to X, the codebase for Wayland is small. This isn't just because Wayland hasn't had enough time to accumulate cruft, it's also because of the design of Wayland. Wayland makes use of a lot of the underlying kernel infrastructure and streamlines the functions it provides. To give you an idea of how the infrastructure of Wayland differs from X:
https://wayland.freedesktop.org/architecture.html
2. Wayland may not be as old as X, but it's not a new project either. Development of Wayland started in 2008, and has been a freedesktop.org project since 2010.
3. Nobody claims Wayland will be perfect, it just has to be better than X to be a worthy successor. Second-system thinking works out fine when the problem space is well understood, and the issues of X appear to be understood just fine.