The list of ICAO countries is staggering (over 100 countries). Each country operating tens of hundreds (depending on the country) of daily flights. Including GA, the number will reach in the thousands. Could you imagine the cost, or who would pay for it, or who could sit and monitor all of those flights? Even if the system would be limited to just the US and UK, that's still hundreds of thousands of daily flights to monitor. Also, there are parts of the world, like the ocean's for example, which don't have radar coverage (or even ATC coverage), so no one is going to know what's going on with the aircraft until the crew makes the mayday call on the radio as the transponder isn't being put to use.
I'm not taking anything away from Sully here, but the outcome of what happened to him was a combination of teamwork. He couldn't have done what he did without the PNF or the rest of the crew. Ditching procedures are listed in the FCOM, and the PNF prepares the aircraft for ditching. One person on the ground can't do that (they need to concentrate on flying the aircraft).
It's reasons like I just mentioned why I think those systems would be impractical.
Edited by Da Bat Man, 18 August 2009 - 11:13 PM.