What is this "appearance in the kingdom" that you are referring to? Verse 27 says Christ Jesus will come, and then reward men.
Let us look sat this verse:
"Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom" (Mt.16 27-28).
One of the meanings of the Greek word translated "coming" at Matthew 16:28 is
"to appear" (
Thayer's Greek English Lexicon).
Therefore, we can understand that the Lord Jesus was saying that there were some of the Apostles who would see the Lord Jesus "appearing" in His kingdom before they died. And that happened at the Transfiguration.
The scriptures tell us that He came in 70AD because we know all the signs he gave in Matt 24, Luke 21, and Mark 13 took place, and we know from secular history what happened to Jerusalem from 66AD - 70AD.
Everything played out exactly how Jesus said it would.
This has not yet happened:
"And there shall be signs in the sun, and in the moon, and in the stars; and upon the earth distress of nations, with perplexity; the sea and the waves roaring; Men's hearts failing them for fear, and for looking after those things which are coming on the earth (oikoumene): for the powers of heaven shall be shaken." (Lk.21:25-26).
Well known preterist author Gary DeMar gave the following definition for the Greek word
oikoumene:
"The case can be made that 'oikoumene' is used exclusively for the geographical area generally limited to the Roman empire of the first-century and the territories immediately adjacent which were known and accessible to first-century travelers. When first-century Christians read the word 'oikoumene,' they thought of what they knew of their world" [emphasis mine] (Gary DeMar, "The Gospel Preached to All the World, Part 3 of 4; The Preterist Archive).
There has never been a time when a judgment came upon a land mass as large as the area of land first century Christians knew of their world.
And since the prophecies of Luke 21:25-26 have not yet been fulfilled then we know with certainity that the prophecy which follows has not yet been fulfilled:
"And then shall they see the Son of man coming in a cloud with power and great glory" (Lk.21:27).
Despite all these facts you assert that He has already returned!