ptrace replaces procfs? Wow, that's a disappointing turn of events. It's also against the flow of thought in the Unix community I knew in 1980. If somebody has any of the old ca-1980 BSD manuals, you should find a BUGS section on ptrace advocating a move to a file-system-like interface. I always assumed ken wrote that little note when he was visiting UCB -- anybody know?
Plan 9 implements process debug via /proc, and several debuggers use that interface -- including, in harvey, gdbstub so we can remote gdb processes. I also implemented an strace-like command by extending /proc/pid/ctl with a few extra commands. In so doing, I made it possible to write strace with a shell script, which was kind of fun.
I always felt the /proc/pid/ctl model in Plan 9 implemented what we hoped to see implemented in Unix to replace ptrace. ptrace, even the new ones, are clunky in the best times.
The file system model is powerful. The strace based on /proc was a few dozen lines.