I haven't yet seen it stated, so I will now, that the the technological capacity to stream live DFDR/CVR data simply did not exist at the time that recording devices were introduced so on-board storage was the only answer.
Technology has moved on in several leaps and bounds since that time. It is now completely feasible to stream live data at relatively low cost.
Surely the answer is that new build transport aircraft should have live streaming designed in (in addition to existing on-board storage) and a later decision can be made, as the existing fleet is retired, whether a live streaming solution ultimately replaces on-board storage.
Is this not making best use of current technology without casting all eggs in one barrel at probably uneconomic cost.