A lot of things descended from PLATO. WELL didn’t, at least directly.
Among the online services I have seen, the WELL has best succeeded in building a community comparable to PLATO’s. Ironically, the WELL has its roots with EIES and Confer; as far as I know, its founders were unaware of PLATO.
But the WELL is an intentional community. PLATO was an accidental one which emerged spontaneously in an environment that had been created for other purposes. In 1970 few suspected that a human community could grow and thrive within the electronic circuitry of a computer. PLATO demonstrated that this is not only possible, but inevitable. (Source)