It is.
Sounds to me like depressurisation,
Possibly an option if we accept the idea that the aircraft was indeed the object detected on radar crossing the Malay peninsula.
but you need to link depressurisation and transponder loss. This could be caused by structural failure taking out antennas, or electric supply loss on a common supply
I wonder if there was a problem with the flight deck oxy supply
Perhaps it's worth asking a suitably qualified engineer for his/her opinion on the proximity (or not) of the crew oxygen bottle to significant electronic/electrical components
. Could a problem with one cause a problem with the other?
Not speculatin', just askin'.....