Nephi's reference to Joseph's promise

In 2 Nephi 25:21, Nephi refers to a promise given to Joseph, Jacob's son

Wherefore, for this cause hath the Lord God promised unto me that these things which I write shall be kept and preserved, and handed down unto my seed, from generation to generation, that the promise may be fulfilled unto Joseph, that his seed should never perish as long as the earth should stand.

What Nephi writes on his plates shall be kept and preserved. This is apparently crucial for the preservation of Joseph's seed that was promised to him by God. We don't find this promise in the Old Testament, but we know it was found on the brass plates, because Lehi quotes it in 2 Nephi 3.

Compare with 2 Nephi 3:15-16

15 And his [Joseph Smith Jr.] name shall be called after me [Joseph in Genesis]; and it shall be after the name of his father [Joseph Smith Sr.]. And he [Joseph Smith Jr.] shall be like unto me [Joseph in Genesis]; for the thing, which the Lord shall bring forth by his hand [The Book of Mormon], by the power of the Lord shall bring my people unto salvation.

16 Yea, thus prophesied Joseph [in Genesis]: I am sure of this thing, even as I am sure of the promise of Moses; for the Lord hath said unto me, I will preserve thy seed forever.

I added the brackets for clarification. "The Lord shall bring forth by his hand" more than just the Book of Mormon, as we know. The restoration of doctrine, priesthood, temple ordinances, etc. But this statement is specifically referring to verses 11-12

and unto him will I give power to bring forth my word unto the seed of thy loins...Wherefore, the fruit of thy loins shall write

In context, the message is clear: Joseph in Genesis received a promise that his seed would be preserved. He also received a promise that his seed would write (Nephi being one of them) and that a Joseph in the latter days would bring this forth. These two promises seem connected as they are expressed both in 2 Nephi 3 and 2 Nephi 25, when Nephi is referring to it. It is another example of internal consistency.

Why can the seed of Joseph be preserved only if the Book of Mormon is preserved and brought forth? I'm not sure I have the answer to that, but I think it has to do with the latter-day gathering of Israel. The seed of Joseph was scattered and would remain scattered and diluted without a regathering. The Book of Mormon is both an instrument and a sign of this regathering (see 3 Nephi 21), causing Joseph to become a "righteous branch" in the House of Israel. This brings me to another example of internal consistency in 2 Nephi 3.

2 Nephi 3:5

Wherefore, Joseph truly saw our day. And he obtained a promise of the Lord, that out of the fruit of his loins the Lord God would raise up a righteous branch unto the house of Israel

Compare with Jacob 2:25

Wherefore, thus saith the Lord, I have led this people forth out of the land of Jerusalem, by the power of mine arm, that I might raise up unto me a righteous branch from the fruit of the loins of Joseph.