Chaumian ecash requires more trust in a single party (e.g. a Cashu mint) while blockchain smart contracts can be designed to decentralize trust and distribute it to the entire network. The question is whether or not SimpleX Chat ltd. will remain a trusted party for other reasons. If the payment system can be made fully trustless, the blockchain approach has merit. Otherwise it'll be an unnecessary complication.

There's no straightforward way to resell these vouchers if I'm understanding the proposed system correctly, so they aren't useful as an investment and won't serve to raise capital for anyone.

I'm not sure this is the best solution. It does seem to solve the stated problems, but it's difficult for a lot of users to understand what it solves and why.