A touch ID is a flawed as a hijacker could place the crew members finger there. Additionally, some aircraft have dead bolts in case the door locking system is unserviceable hence, the only way to gain access to the cockpit is for the door to unbolted from the inside.
What concerns me most, is that essentially, the government is saying - "we don't trust pilots".
Surely cabin crew are just as likely to be depressed or psychologically disturbed, stressed or what ever. This new policy says- "Pilots - Untrustworthy - Cabin crew trustworthy therefore problem solved.(said with sarcasm)