@p.j.m.
Who's going to pay for the monitoring, and storage (and retrieval) of all this data, and who's going to monitor it in case 1 aircraft stops transmitting (for whatever reason), and even if they did detect an anomoly, who are they going to notify?
The Spidertracks product linked above looks like it costs $1000 - $2000 per aircraft for hardware. And appx $1.60 per hour operating cost. It is a small standalone product that would provide a tamper proof method of continuous flight tracking.
Seems like a smart and minimal cost idea - especially now that we know you can apparently easily disappear a 777 ...
I suspect something similar will be required on all commercial aircraft in a very short time - even if as a backup system.
Smart airlines should be stepping all over each other to see who can announce and install these quickest - as a safety and PAX security tool.