Some rewrites can be easy because a better technology exists that makes it easy. For example, you might have a NoSQL DB with lots of hand-rolled queries, and t

Some rewrites can be easy because a better technology exists that makes it easy. For example, you might have a NoSQL DB with lots of hand-rolled queries, and then you get a query language for it, so now you can rewrite a great deal of code. You still hand-roll transactions because eventual consistency, CAP, and all that, but then you get a SQL DB that... gives you all of that anyways but with good ole SQL (with eventual consistency, not ACID, but hey, CAP), so now you can rewrite the queries and the transactions, and maybe that's the lion's share of the app, so you've essentially rewritten the app. What about the REST layer? Well, you get inspired by PostgREST and write a similar tool for your new DB and there's nothing left of the original.