Much debated question but if we run with your definition, then A2A adds communication capabilities alongside tool-calling, which is ultimately a set of programmatic hooks. Like "phone a friend" if you don't know the answer given what you have available directly (via MCP, training data, or context).
My assumption is that the initial A2A implementation will be done with MCP, so the LLM can ask your AI directory or marketplace for help with a task via some kind of "phone a friend" tool call, and it'll be able to immediately interop and get the info it needs to complete the task.
To my understanding of this protocol it looks like it's an entity exposing a set of capabilities. Why is that different and complementary to an MCP server exposing tools? Why would you be limited to an "everything agent" in MCP?
I am struggling to see the core problem that this protocol addresses.