Question: Why did Joseph describe the United Order in revelation as "everlasting" and "immutable and unchangeable" until Jesus comes?

The United Order is an "everlasting" covenant because it comes from God, reflects his purposes, and is attended by promised blessings for all who obey

This does not mean—just as with biblical examples which use identical language—that "everlasting" is a prophecy about its practice or implementation.

The relevant scripture reads (color emphasis added for clarity):

1 Verily I say unto you, my friends, I give unto you counsel, and a commandment, concerning all the properties which belong to the order which I commanded to be organized and established, to be a united order, and an everlasting order for the benefit of my church, and for the salvation of men until I come—
2 With promise immutable and unchangeable, that inasmuch as those whom I commanded were faithful they should be blessed with a multiplicity of blessings;
3 But inasmuch as they were not faithful they were nigh unto cursing.
4 Therefore, inasmuch as some of my servants have not kept the commandment, but have broken the covenant through covetousness, and with feigned words, I have cursed them with a very sore and grievous curse.
5 For I, the Lord, have decreed in my heart, that inasmuch as any man belonging to the order shall be found a transgressor, or, in other words, shall break the covenant with which ye are bound, he shall be cursed in his life, and shall be trodden down by whom I will;
6 For I, the Lord, am not to be mocked in these things—(DC 104:1-6)

Several points need to be made:

the practice of the Order is not prophesied to be "immutable and unchangeable." Rather, the Lord says that the promise is "immutable and unchangeable"—and, that promise is that "inasmuch as those whom I commanded were faithful, they should be blessed with a multiplicity of blessings.
the United Order is to be everlasting—that is, it is always the Lord's highest law. Temple-worthy Latter-day Saints promise to observe the law of consecration. They are not, at present, commanded to enter the United Order, but covenant to do so if asked.
the Lord makes it clear (verses 3-6) that some might break the covenant, and suffer the penalty. Thus, failure to live the law is not failure of a prophecy, but failure to live a commandment.

Biblical parallels: similar uses of the term "everlasting" that describe the importance and efficacy of certain commandments or ordinances

There are similar uses of the term "everlasting" that describe the importance and efficacy of certain commandments or ordinances. Yet, Christians do not believe they are bound to continue to observe these ordinances and covenants at all historical times. For example (emphasis added in all cases):

Aaron and the Levites are given an "everlasting priesthood throughout their generations" (Exodus 40:15, see also Numbers 25:13). Yet, modern day Christians (like many of our critics) do not seem to believe that the only legitimate priestly authority persists with Levitical descendants, or that such descendants currently enjoy divine sanction.
Circumcision is described as "a token of the covenant betwixt me and you" that "my covenant shall be in your flesh for an everlasting covenant." Those who are not circumcised "shall be cut off from his people; he hath broken my covenant" (Genesis 17:10-14). Yet, modern Christians do not believe that circumcision continues to be binding or necessary.
Likewise, the "bread for a memorial" is commanded to be "set...in order before the Lord continually," since it is "taken from the children of Israel by an everlasting covenant" (Leviticus 24:8). Do the critics likewise believe that this ought to be continued in unbroken succession to the present for it to be a valid commandment from God?

Notes

Question: Why did Joseph describe the United Order in revelation as "everlasting" and "immutable and unchangeable" until Jesus comes?