What will happen with the next generation of ATSC only DVD Recorders when set up to RF connections on current model digital to analog cable converter boxes?

I know, the advice will be to use other outputs connected to DVD Recorder inputs.

My example is the Comcast Motorola DTC700 digital converter box. The only outputs are RF and composite. In my setup one Panasonic DVD recorder is connected to the DTC700 RF output, the other is connected to the DTC700 composite output.

Once DVD Recorders have ATSC only tuners the cable box RF output will become useless as a signal source.

Yes I suppose it would. To feed more than one DVDR off of your STB you'd need some type of composite amp/splitter. That is one composite in, 4 out.
I've personally found that splitting the composite output, with say a simple Y cable, makes the picture much darker. You'd need a amp.
Note because of low power NTSC broadcasts I'm not sure they will not fully drop the NTSC part, but I suppose they could. The Samsung 260 is a example of a tuner that is only digital, no analog tuner, and the Polaroid DVDR actually has a separate RF input for digital and analog tuners. It would sure be easy for Polaroid to just eliminate NTSC part on that unit.