This meant that X11 was open to applications to interact with it and implement functionalities that otherwise X11 didn't offer. Also, X11 allowed functionality to be implemented by multiple, small programs with one purpose: window manager, compositor, screen recorder, screen locker, screensaver daemon, clipboard manager, etc.
Wayland is closed in the sense that all the functionality needs to be incorporated in the compositor, since otherwise the protocol lacks of the required interfaces for other applications to communicate.
Sure, from a security/privacy prospective it's good that any application can't see or interact with the content of other windows. But, we are not on macOS or Windows! Us, and with us I mean us Linux users, trust 100% the software that we run on our machine, because we trust the open source community! Thus I don't care if any application may access the screen, because I only run trusted software.
I prefer that an application may do so, since by doing so it can implement functionalities that are not possible on proprietary operating systems! This to me means being open, not just having the code available, but also being open for interaction between any piece of software on the computer.
To me it seems that Linux is going into a wrong direction because it makes assumptions that make sense only in the context of proprietary operating systems like macOS/Windows. We choose to ditch these systems for an open OS not to have these issues! I don't want to have the stupidity of these OS ported on Linux...