Does not compute... Ship of Theseus is just regular old development of course it never gets done but new features aren't put on hold.

I mean like “we want to replace X with Y”. Y incrementally starts replacing X, but 100% migration is never achieved, meaning double the API surface area exists indefinitely.

Because the migration doesn’t block new features, that means the org gets tired and reallocates the effort elsewhere before it’s ever done, with no immediate consequences. Rinse and repeat.

I think you've not witnessed Ship of Theseus, but "build Ship2 next to Ship1 and start using Ship2 while Ship1 is still being used and keep saying you're going to migrate to Ship2 eventually but meanwhile Ship1 and Ship2 diverge and now you have 2 ships".

I recently witnessed this mess and it is an enormous mess. Don't build Ship2 in the first place. Instead, replace Ship1's mast and sails, and rudder etc until you've replaced all the parts in Ship1. That's the SoT approach.

Right but how do you replace the masts? Don’t you have to build mast2 and then tear down mast1 if you want to have continuous propulsion?

Yes, can you see how that's quite different from building a second ship?

In my comment, X and Y are different masts, not different ships.

I understand now.