This is the kind of absurd, foundation-shaking statement that, as someone who's been coding since I was 7 (in 1987..) feels purely nauseating. Although I sort of know the answer, my first question is how did we reach the point where something so fundamental could be so insecure and distributed to so many people?
In the original Unix security model, there was no security concern with this (except maybe for chroot environments): it didn't allow a process to do something it couldn't otherwise do, since all processes owned by a uid had exactly the same rights. Now that we've started sandboxing user processes in various ways on macOS and Linux, that's no longer the case, and we suddenly need to crack down on useful tools like strace and gdb.
And since the two processes already run as the same user, in the original model there's nothing the target can do that the debugger cannot also do, so this was not a privilege escalation path.
I called ptrace() on your pid, therefore I am your debugger now.
Unix security model was never meant to protect user processes between themselves, it only meant to protect system from user apps, which is a bad model these days, where basically every device has a single user, and the real threat is user apps hacking and stealing each others data.