Dead Man's Chest has Jack Sparrow carrying out the following plan, in precisely that order:
- Steal the drawing of the key to Davy Jones' chest
- Locate and steal said key
- Retrieve and open the chest
Now if for some reason the key cannot be forged again, that makes a lot of sense. But for a complete ignorant of blacksmithing like I am, it doesn't look that complicated a metal shape to reproduce:
It would seem much less risky to steal the drawing, retrieve the chest, and then settle down somewhere and try to re-create the key. Holding fort is easier if it's on land, since Jones can't come ashore. I can see two obstacles, but also an attempt at overcoming them:
- the drawing isn't a 1:1 scale. Fiddling with sticks of various lengths could give you a better idea of the key's actual length.
- no forge/blacksmith available. I don't know if Tortuga has a forge, but it doesn't seem that much of a reach. And if it boils down to blacksmith's skill, Jack gets a pretty talented one quite soon, when William Turner meets him again.
Out-of-universe it wouldn't be very thrilling to watch pirates elsewhere than sea, trying out different metal molds, but in-universe it sounds way safer. Why wouldn't they do so?