Only one thing I disagree with - bandwidth costs. The FDR [apparently] records at about 6kB/s (I'm trying to find a source to back that up), so that's 6kB/s per aircraft in satellite data costs. There can be up to 11,000 aircraft flying at any one time, so that totals 66MB/s of bandwidth required globally. I don't believe lack of bandwidth is a factor.
Bandwidth is apparently between 5 and 7 USD per MB, so about 4c per second. In comparison, and A320 consumes about 665 gallons of fuel per hour, equating to about 29c per second. It's not an insignificant increase in cost. Along with that, the cost of equipment, certification, etc is why we haven't seen it yet, though I believe there are working groups.
There is also the issue that it will always be possible for the flight crew to pull the breaker on the transmitter, much like they can pull the breaker on the CVR (and probably the FDR), so that doesn't solve the issue when it comes to a malicious agent.