what is to stop the current generation of CVRs being replaced with solid state devices capable of capturing everything said from ground power on to beyond final shut down on a maximum-range flight
Cost...
Modern CVRs and FDRs are solid-state, basically a load of memory chips in a sealed titanium container with all the support electronics needed to write to them on the outside. On recovery the external electronics is discarded and the container is cut open and connected to a reader in the lab. The whole lot is inside the black box proper, so all the other fire/impact/water defences are still there. It's more reliable than tape because it has no mechanical component to wear out (or tape to snap/jam). It's also not cheap, although the incremental cost for a new build to put in 8 hrs instead of 2 hrs is probably quite small.