The long-term sustainability of the plasma system would be contingent upon the establishment of a secure and democratized governance system. The system would most likely move from the control of its founders to an "on-chain governance model," in which stakeholders including validators, developers, and possibly stablecoin users would be able to propose and vote for upgrades to the protocol. This would encompass controls for key stakes such as fees, validator rewards, and treasury.
Transparency and safe handling of upgrades are of utmost importance. A process, which could include signaling periods and timelocks, will help minimize issues inherent to centralization and execution. The real value that appears to have the largest potential is not just mimicking what is currently possible within DeFi, but leveraging new "killer apps" that are expressly adapted to this stablecoin-centric ecosystem, such as completely autonomous cross-border payroll systems, real-time B2B invoice settlement, and programmable loyalty systems.
Ultimately, the idea is that the ecosystem itself drives the development process forward, such that it will continue to evolve as a truly neutral public good secured by members for the singular mission of value exchange worldwide.