A full rewrite absolutely should not be taken lightly (if ever). It's very much a last resort and something that requires deliberation and a clear path to succ

A full rewrite absolutely should not be taken lightly (if ever). It's very much a last resort and something that requires deliberation and a clear path to success. You're spot on with your rules - nothing new; nothing lost. I had a similar experience, sans the cabin. I was hired onto a startup that already had a functioning app in the wild. The API was written by one of the founders, who is not a developer in the professional sense, and holds my undying respect. As an early-stage startup with lots of ideas, we needed to move fast, and I wasn't going to be able to do so with the existing code base. I stocked the fridge, locked myself into my tiny Brooklyn apartment, and got to work. I started by logging all requests to the API in order to ensure I had all the necessary endpoints covered. Then I wrote integration tests - acting as an HTTP client - for the entire API. About a week or so later, once the rewrite was finished, I added automated tests that compared the output between the two APIs, and once those matched perfectly, ran it live beside the original API (sending requests to both) and compared results from real requests to ensure there were no discrepancies. Besides a couple very small bugs after the switch, it went very well. The user base was none-the-wiser, besides the sudden uptick in features after the rewrite. The startup was relatively successful (acquired), and I still work with those guys from time to time.