I believe that Kel-Tec has plenty of reason to offer the RDB, beefed up to 308. They aren't concerned with competing with their RFB because the 308 version of the RDB will replace the RFB. Why replace a rifle that can't be built fast enough to meet demand with a similarly featured rifle that will require significant engineering and tooling costs? Because the 308 RDB will be much faster and easier to build. They can build many more of them, and their profit margin on each will be higher. It's a business thing.
Step 1. Collect underpants.
Step 2. ?????
Step 3. Profit.
Why would Kel-Tec redesign the SUB-2000 to make the Gen 2 SUB-2000? Because... better, faster. They added more features that customers wanted and they can build them faster so twice as many are sold every week. More guns for us. More money for them. Everybody wins.
And nobody is thinking they can put a 308 barrel on an RDB and ship it. Obviously, the case is much longer so the action will need to be scaled up accordingly, but the increased power will also necessitate a more solid frame as well. I suspect they'll try to reuse some of the trigger group components, but everything else would be scaled up and beefed up as needed. However, that's not nearly the engineering feat that a completely new design would be.
I wouldn't expect a 300 BLK RDB, must as I'd love to have one. It seems that George is soured on the 300 BLK. Kel-Tec has always had a strong preference for well established cartridges and it seems that lately, whenever they talk about something outside the box, it's not 300 BLK.