The hardest part is network effects.

Consider that the largest payment card network on Earth (China UnionPay, 7 billion cards) - decided it was easier just to bootstrap acceptance in the US by a partnership with Discover rather than connecting directly to merchants.

If you want a new scheme to work, distribute something like social security and welfare cheques through it. That immediately forces broad acceptance.

Isn't US EBT card on the same payment network as credit cards? That doesn't count as an independent system. The same set of "they" as Visa/Mastercard gets the fee.