[hinkley]

I would argue that the rewrites help when the information architecture for the original code is proven to be wrong, and there is either no way to refactor the old code to the new model, or employee turnover has resulted in nobody having an emotional attachment to the old code.

That said, to slot in a new implementation you often have to make the external API very similar to the old one, which can complicate making the improvements you're after.

[bluGill]

> there is either no way to refactor the old code to the new model

That doesn't happen. Write facades as needed. Even if they are slower than everything else write the facades so you can keep in production all along.

[hinkley]

If you get the object ownership and the internal state model wrong (information architecture) facades don't help you.

You can't put an idempotent or pure functional wrapper around a design that isn't re-entrant and expect anything good to come from it. IF you get it to work, it'll be dog slow.