FXBs will continue to be supported long-term within the Frax ecosystem. While they will redeem into Legacy FRAX for now, the intention is to back future FXB issuances with RWA-based collateral once legal and technical infrastructure is in place. also it worth to mention that liquidity incentives for FXB pools will be there to support these assets.
Also regarding the veFRAX rewards as we discussed it in our FIP-428 will be in sfrxUSD and as you mentioned they will be generated by minting frxUSD using protocol revenue and then staking it for sfrxUSD.