Origin and purpose of the dagger in "Star Wars: The Rise of Skywalker"

Objects like these are called Macguffins.

They are designed to move a plot along somehow, giving reasons for characters to go from one place to another, often times being mysterious.

A MacGuffin (a.k.a. McGuffin or maguffin) is a term for a motivating element in a story that is used to drive the plot. It serves no further purpose. It won't pop up again later, it won't explain the ending, it won't do anything except possibly distract you while you try to figure out its significance. In some cases, it won't even be shown. It is usually a mysterious package/artifact/superweapon that everyone in the story is chasing.

This movie had 4(!!) Of them like a bad fetch quest in a game.

1. The dagger
2. The translation
3. C3-PO (tied in with 2)
4. Two different cubes
5. Rey herself (living macguffin)

There is no relevance to the dagger besides moving the plot along and no second thoughts about space metal oxidation.

I highly suggest reading about the different typed of macguffins on the provided links.

There are only a couple of possible conclusions...

Ochi, also known as Ochi of Bestoon, was a humanoid alien male who was a Sith loyalist serving under the resurrected Dark Lord of the Sith and Galactic Emperor Darth Sidious during the New Republic Era. Sometime after 15 ABY, he was sent to retrieve the granddaughter of Darth Sidious—Rey—from her parents. Ochi killed both of them when they refused to give him Rey's location. Sometime later, he was tracked down by Luke Skywalker and Lando Calrissian to Pasaana, where Ochi became stranded and eventually died.

Ochi was an assassin and Sith Loyalist who was assigned to find Rey and bring her back to Darth Sidious when she was a child. The Passanna plot in film allowed Rey to find Ochi's dagger and we may assume that this is HOW Ochi would find his way to Sidious/Exogal, meaning that he needed the dagger in order to find one of the Wayfinders (Sith Holocrons) to actually get to Exogal.

So the set-up with the Sith Dagger seems to rely on time and/or religion, as we may also assume with the aid of the Star Wars Visual Dictionary that The Sith Loyalists and/or Sith Eternal would of been responsible for creating such a limited device that, in theory, would become useless, should the wreckage naturally erode or plunge further into the sea.

This could be about either believing in a time window being spiritually important to their plan or simply a way to protect the Wayfinder/Sidious, as it was not the only one and therefor not the only way to find Exogal, and perhaps they didn't want it to be exposed out there for too long, for just anyone to find?

The Sith Eternal was an organization of cultists loyal to the Sith religion who continued the Sith belief and sought to resurrect its tradition. The Sith Eternal were followers of the Dark Lord of the Sith and Emperor, the Sith Lord Darth Sidious...

However, while there is not much yet known about The Sith Loyalist, or more importantly, the cultists: the Sith Eternal, it might be possible that the Death Star wreckage might defy physics due to some members of cult using the Force to keep it in place.

It has begun to come to light that some of the Sith Loyalists, Sith Eternal (Sith cultist), or Alchemist like Albrekh may have been dabbling in the dark side of the Force. Even the Knights of Wren have been revealed to be "mildly" Force sensitive, let a lone we don't know what The Sith (spirits) or Palpatine was also able to do. So this might be one possibility.

When Ochi was given his mission to fetch Rey, the dagger was created by the Sith to give him the information he needed to locate Exegol in order to deliver her.

The runic inscription has the GPS coordinates of where to stand. The shape points to where the vaults are.

The existence of the wayfinder and the information it contained were too sensitive to create a standard map, so it was obfuscated so that if the dagger was found it wouldn’t be useful.

The dagger and its relationship to the wreckage aren’t a problem if it’s new. There’s no reason to think it’s ancient, so it’s positioning doesn’t need to last long - just until he completes his mission.