Can files be protected AFTER ransomware starts running?

The post author observes that most advice regarding ransomware focuses on prevention (antivirus, backups, etc.) and wonders if there are ways to protect files once malware is already running on the system. They mention several possible approaches such as file permission locks, required authentication for file writing, and detection of mass encryption attempts. The author questions the feasibility of these "last line of defense" approaches and seeks the opinion of security professionals.