I think that's a passing artifact of the current phase of AI development we find ourselves in. Capabilities (note how I'm not saying 'model intelligence', as I think various agentic flows and robust scaffolding/harnesses can lead to capabilities growth that goes beyond model intelligence plateauing) will continue to improve such that 'beginners' will gain greater and greater leverage.

If by 'marginal AI user' you mean a user who leverages AI tools to enhance the marginal utility of their labor or tasks (by making them more productive or efficient, broadly defined), then I do think that user archetype definitely exists.