So, now our intruder has shell access. They can do tons of damage already, without privilege escalation.
They can then, if they so desire, try to leverage privilege escalation. They need a password, either the login (sudo), root (su), or with your method root (target_pw). So, either way, they are one password away from root access. As with shell access, many exploits (code injection) do not require determining the password. The ones that do gain the password work with all 3 (sudo, su, or target_pw).
Your problem is that you are assuming that the only exploit or method of privilege escalation is via password compromise, and, sadly that is simply not the case.