Just so you know how much data we're talking about, I did a quick check of a DFDR file I have from a six hour flight (I was investigating a strange engine anomaly on the flight). Six hour flight is a 150 meg file, or roughly 25 meg per hour (and that was an older DFDR, new models record even more). Multiply that by the thousands of airplanes in the air at any time and you're going to quickly overwhelm the available satellite resources. So $billion$ of new capability would be needed for the downlink.
I think a far better path would be improve the availability and 'findability' of the data recorders, along with some mandatory automated downlink of aircraft GPS position data.