And it doesn't solve the root problem of duress on a crew member in the passenger cabin. If terrorists know they can either force a finger/eye onto a reader or cut off a finger and gain immediate access to the cabin, then you can grab anyone in the cabin at any time. Now passengers post 9/11 would be apt to try to stop you, but under the right circumstances a hijacker could gain access to the cabin. You can try to force someone to give you the code as well, and under the threat of injury/death (or actual injury) a pilot might comply. Combine that with needing to enroll all crewmembers (revocation of access when a pilot/flight crew member leaves, adding them when they join, etc.) and it's really not feasible.

When you compare the number of attempted hijackings versus the number of pilot suicides, it doesn't make sense not to give the captain a way to lock out the cabin crew if he's conscious. Now maybe they enhance this system where lockout requires a switch to be pressed by both pilots at the exact same time within the cabin to prevent an emergency code (and wide enough apart where one individual cannot press both switches at once).