Dan,
Good thought. I guess it all comes down to deciding upon an 'acceptable level of safety'.
Personally I feel decidely uneasy in flight with the TCAS on the blink. It is of course fallible, I see one particular turboprop on my flights regularly and he seems to be always in stealth mode even when radar identified.
Airmanship would dictate that each defence mechanism be used to full extent and then follow roughly your ideas on the OMEL. I dont like giving the holes in the cheese too much chance to line up!
The law makers on the other hand would reakon that we dont need it as we have our sight. Next thing radar altimiters and GPWS will be out. I mean we can all see the ground in between the clouds cant we!!
Enough rambling. Agree with you.
Merry Christmas!