So reading through many of the posts, cannot see many theories that seem to fit what appears to have happened.
No debris spotted (still)
No mayday calls
No messages sent from aircraft via ACARS etc.
No transponder info
Which would appear to equal
Steep descent into water (small debris field and needle in a haystack)
No transmissions made (why?)
ACARs switched off or disabled (why?)
Transponder switched off or disabled (why?)