There is no doubt that when the people of Paradise enter Paradise, they will abide therein and enjoy its pleasures for ever and ever. This is the belief of Ahl as-Sunnah wa’l-Jamaa‘ah, that the people of Paradise will enjoy its pleasures and abide therein for eternity, and the people of Hell will be punished in the Fire and abide therein for eternity. Allah, may He be exalted, says (interpretation of the meaning):
“But those who believe (in the Oneness of Allah — Islamic Monotheism) and do deeds of righteousness, We shall admit them to the Gardens under which rivers flow (i.e. in Paradise) to dwell therein forever. Allah’s Promise is the Truth; and whose words can be truer than those of Allah?”
“Verily, Allaah has cursed the disbelievers, and has prepared for them a flaming Fire (Hell).
Wherein they will abide for ever, and they will find neither a Wali (a protector) nor a helper”
[al-Ahzaab 33:64, 65].
Al-Bukhaari (4730) and Muslim (2849) narrated that Abu Sa‘eed al-Khudri (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Death will be brought in the form of a black and white ram, then a caller will call out: O people of Paradise! They will crane their necks and look, and he will say: Do you recognize this? They will say: Yes; this is death – and all of them will have seen it. Then the call will go out: O people of Hell! They will crane their necks and look, and he will say: Do you recognize this? They will say: Yes; this is death – and all of them will have seen it. Then it will be slaughtered, then it will be said: O people of Paradise, it is eternal and there will be no death. O people of Hell, it is eternal and there will be no death.” Then he recited the words: “And warn them (O Muhammad) of the Day of grief and regrets, when the case has been decided, while (now) they are in a state of carelessness” – those who are in a state of carelessness on the people of this world – “and they believe not” [Maryam 19:39].
This idea that you mentioned in your question, that “nothing is eternal except Allah” is an idea for which there is no basis in Islam; rather it is a false idea according to the consensus of the Muslims, who are unanimously agreed that the people of Paradise will abide therein forever and ever, and there will be no death, and the same applies to the people of Hell, those disbelievers who are its people.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
The early generations of this ummah and its imams (leading scholars) are unanimously agreed that among created things there are some that will never cease to be. They are Paradise, Hell, the Throne, and so on. No one suggested that all created things will cease to be except a group of innovators among the People of the Book. This is a false idea.
End quote from Mukhtasar al-Fataawa al-Masriyyah (1/169).
Here we should point out two things:
that the eternal existence of some created things which Allah has decreed will remain forever in the Hereafter will have been preceded by non-existence. As for Allah, may He be glorified and exalted, He is First, with no beginning, and Last, with no end.
that the eternal existence of created beings is a gift from Allah. Just as the created being only exists because Allah created him, and he did not bring himself into existence on his own or with the help of any other created being, similarly the continued existence of creatures in the Hereafter is a gift from Allah to them, and will only be because of His power of creation and His will and decree.
This shows us that the continued existence of all creatures and their eternal existence in the Hereafter in no way resembles the eternal existence of the Creator, may He be glorified, and that that which is unique to Allah, may He be exalted, in that regard is not only that He will remain forever and never cease to be; rather the Creator, may He be exalted, is unique in that in a manner that befits Him; He will exist forever in a perfect manner as befits the Creator, and the created being will exist in a manner that befits him as having been created by Allah.
Shaykh al-Islam (Ibn Taymiyah – may Allah have mercy on him) said:
We must be certain that there is nothing like unto Allah in His essence, in His attributes or in His actions.
End quote from Majmoo‘ al-Fataawa (13/379)
With regard to His eternal existence, Allah, may He be glorified, possesses an attribute which no one shares with Him, which is the attribute of being the Last. The Prophet (blessings and peace of Allah be upon him) said: “O Allah, You are the First and there is nothing before You; You are the Last and there is nothing after You. You are the Manifest (al-Zaahir) and there is nothing above You; You are the Hidden (al-Baatin) and there is nothing beyond You.” Narrated by Muslim (2713).
Shaykh al-Islam (Ibn Taymiyah – may Allah have mercy on him) said: So there is nothing before Him, nothing after Him, nothing above Him and nothing beyond Him.
End quote from Majmoo‘ al-Fataawa (16/359).
He also said:
He is the First before Whom there is nothing; he is the Last after Whom there is nothing; he is the Ancient of Days, the Eternal, the Everlasting Who will never cease to be.
End quote from Dar’ Ta‘aarud al-‘Aql wa’n-Naql, 1/69
Ibn al-Qayyim (may Allah have mercy on him) said:
Allah’s being the First means that He existed before the beginning of all others besides Him, and His being the Last is affirmed and means that He will outlast all others besides Him. His being the First means that He was there before all things and His being the Last means that He will remain after the end of all things. His being the Manifest implies that He is above all things; the meaning of Manifest implies highness and being above, for that which is manifest of anything is that which is above and encompasses what is inside. His being the Hidden means that He is aware of all things, in the sense that He is closer to a thing than itself. This nearness is not the nearness of the lover to his beloved; the two are quite distinct and different.
What these four names and attributes convey is encompassing and being aware, and that is in two senses: time and place. His being the First and the Last encompasses near and far. Everything that is before something else is within the framework of His being the First, and everything that comes after something else will end within the framework of His being the Last. Thus His being the First and the Last encompasses the early and the late. His being the Manifest and the Hidden encompasses everything that is manifest and everything that is hidden. Nothing is manifest but Allah is above it, and nothing is hidden but Allah is beyond it. Nothing is first but Allah is before it, and nothing is Last but Allah is after it. The First refers to His prior existence from eternity; the Last refers to His everlasting existence; the Manifest refers to His highness and greatness; and the Hidden refers to His closeness and nearness. He preceded all things by being the First, and will remain after all things by being the Last; He is above all things by being the Manifest, and He is nearer than all things by His being the Hidden. No heaven can hide another heaven from Him, no earth can hide another earth from Him; nothing above can hide from Him anything that is below. Rather the hidden for Him is manifest, the unseen for Him is seen, what is far for Him is near, and what is secret for Him is known. These four attributes refer to the pillars of Tawheed. He is the First yet He is the Last, and the Last yet He is the first; He is the Manifest but He is Hidden, and He is the Hidden yet He is the Manifest. He has always been and always will be, Manifest and Hidden.
End quote from Tareeq al-Hijratayn (31).
And Allah knows best.