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Most of the time applications can read each other's memory, so protecting the display does not make much sense.

Basic X client isolation (not using XACE just Xsecurity) would have worked for sandboxed applications with some minor changes (allowing access to some basic modern extensions, I had a local patch for this once). There is really not fundamental issue in X that could now allow isolation of clients.

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> Most of the time applications can read each other's memory, so protecting the display does not make much sense.

Unless you're running everything as root, applications can not read eachothers memory.

Of course they can! How do you think a debugger works?

Because a debugger launches the target process as a child process and thus has access to its memory?

/proc/sys/kernel/yama/ptrace_scope is usually set to 1 on non-Selinux distros.

Without this flag it can also attach to existing processes and on my Debian this is set to zero.