Our hope is that by building up a set of these patterns, and making them available to agents via MCP, it will be as easy as possible to get things right.
No offense, but you failed here. What if nothing gets returned? Now you're basically just writing code. Debatably even that example you're just writing code. An onLoad callback with array indexing and child access? This is just JavaScript wrapped in XML.
I am not sure about prior art but the ability to mix built-in and user-defined components, and fluidly refactor the mixture, feels very powerful to me.
I wrestled with how much history to bring into this because I want to look forward not back. The purpose of the announcement is to encourage people to try the tool and find out for themselves if it is a productive way to build the user interfaces they need.