Good taxonomy but in practice these compose more than they compete I think..
Production agent stacks already layer MCP for tool access on top of RAG for context retrieval, with RLM-style orchestration wrapping the whole thing. The question nobody answered cleanly to me yet is which layer owns state. I wrote about this decomposition (1) and RAG's specific failure mode with sequential data (2).
(1) https://philippdubach.com/posts/dont-go-monolithic-the-agent...
(2) https://philippdubach.com/posts/beyond-vector-search-why-llm...