There's a subtle but important thing to note here. My bet is management didn't hate that you did a POC. They hated that you DIDN'T also do a writeup.
It's not about this vs that. Meet the minimum requirements before you go doing extra.
The documents are hard specifically because not you have to consider alternatives, or the feasibility of executing. What security concerns did I cover? How much time do I need to spend convincing team y to add this feature?
The demo is always the easy 50%, and never the hard parts that cause you to spend two years building alignment and executing.