[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.