It is always a mistake to choose one verse from scripture and lift it away from the surrounding verses which takes it out of context. The discussion in 2 Pet. 3 is of the day of judgment that was coming upon that generation as Jesus had promised in Matt. 24:34.
"Verily I say unto you, This generation shall not pass, till all these things be fulfilled." (KJV)
The question Peter was answering was when that day of judgment for that generation would happen. Peter wrote this letter approx. AD 64/65, and it had been about 34/35 years since Christ's crucifixion and His prophesy given to His disciples on the Mt. Olives.
"7 and the present heavens and the earth, by the same word are treasured, for fire being kept to a day of judgment and destruction of the impious men.
8 And this one thing let not be unobserved by you, beloved, that one day with the Lord [is] as a thousand years, and a thousand years as one day;
9 the Lord is not slow in regard to the promise, as certain count slowness, but is long-suffering to us, not counselling any to be lost but all to pass on to reformation,
10 and it will come -- the day of the Lord -- as a thief in the night, in which the heavens with a rushing noise will pass away, and the elements with burning heat be dissolved, and earth and the works in it shall be burnt up." (S Pet. 3:7-10, YLT)
Jesus had told the signs to watch for the destruction of the temple (Matt. 24:2ff). The disciples recognized that a second destruction of their temple would mean another end of another age, specifically the Mosaic age / covenant.
Peter was using the OT covenant language of "heaven and earth" which God promised in Isa. 65/66. The new heavens and new earth was not literal, but was a compound metaphor for the Messianic covenant of the gospel of Christ. Deu. chap. 4 makes that language clear that it was the contract / covenant between God (heaven) and Israel (earth).
"Now therefore hearken, O Israel, unto the statutes and unto the judgments, which I teach you, ..." (Deu. 4:1, KJV)
The subject / context was of the statutes and laws set before them.
"I call heaven and earth to witness against you this day, ..." (Deu. 4:26, KJV)
Moses repeated it again in Deu. 30:19:
"I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing:..." (KJV)
What would witness against them but the laws and statues of the covenant God had made with them? Heaven and earth was a metaphor for the old covenant. The new heavens and new earth was therefore the new covenant. Heb. chap. 8 made it very clear that the old covenant was ready to pass away when that book was written about AD 60 -65.
"In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away." (Heb. 8:13, KJV)
After waiting for the destruction of that temple for 34 or so years, the people were asking Peter, "When?" When was that day of judgment coming? The prophesy was still future to them, and is why the texts of the NT of the 1st century AD are written in the future tense. The temple was still standing when the book of Hebrews was written (Heb. 8:4).
The temple is not standing today, as we know it was destroyed in AD 70 by the Romans, so the prophesy is not still future to us.
Peter's language is OT judgment language which God used to warn the wicked of His soon coming in a "day of the Lord," and his use of this language is the warning of the melting of the elements that Ezekiel had prophesied - not the Greek elements of earth, wind, fire, and water; but the doctrinal elements and teachings of the old covenant which is what the Greek word "stoicheion" means (Strong's Gr. 4747) (1)
"18 Son of man, the house of Israel is to me become dross: all they are brass, and tin, and iron, and lead, in the midst of the furnace; they are even the dross of silver.
19 Therefore thus saith the Lord God; Because ye are all become dross, behold, therefore I will gather you into the midst of Jerusalem.
20 As they gather silver, and brass, and iron, and lead, and tin, into the midst of the furnace, to blow the fire upon it, to melt it; so will I gather you in mine anger and in my fury, and I will leave you there, and melt you.
21 Yea, I will gather you, and blow upon you in the fire of my wrath, and ye shall be melted in the midst therof.
22 As silver is melted in the midst of the furnace, so shall ye be melted in the midst thereof; and ye shall know that I the Lord have poured out my fury upon you." (Ezek. 22:19-22, KJV)
This was the prophesy of the second destruction of Jerusalem which was carried out in the siege by the Romans in AD 70. It is the same prophesy Jesus warned of in Matt. 24, Mark 13, and Luke 21.
"20 And when ye shall see Jerusalem compassed with armies, then know that the desolation thereof is nigh. 21 Then let them which are in Judaea flee to the mountains; and let them which are in the midst of it depart out; and let not them that are in the countries enter thereinto.
22 For these be the days of vengeance, that all things which are written may be fulfilled." (Luke 21:20-22, KJV)
It is the same prophesy Jesus spoke against Jerusalem in Matt. 23.
"37 O Jerusalem, Jerusalem, thou that killest the prophets, and stonest them which are sent unto thee, how often would I have gathered thy children together, even as a hen gathereth her chickens under her wings, and ye would not! 38 Behold, your house is left unto you desolate." (KJV)
Peter's point in 2 Pet. 3:9 was God's appointed time would not be slack, but at His discretion, at just the right time. Some of the people had been making the mistake that their ancestors had made when they said that all continues the same (2 Pet. 3:4; Jer. 17:15, 27; Ezek 12:22-23), and Peter was reminding them that other days of judgment had come in old time when they least expected it.
When reading the scriptures we must always keep the first audience perspective.
For more on 2 Pet. 3, see the post at my blog, "Frequent Mistakes - Part VI: The End of the Workd, or...?" here: ShreddingTheVeil. See also "A Day is AS A Thousand Years" - ShreddingTheVeil.
(1) stoicheion: one of a row, hence a letter (of the alphabet), by ext. the elements (of knowledge) Source: Biblehub