Actually, I believe his demons will be tormented forever as well. They were created with immortal spirits, whereas man was created with an immortal spirit, but when man sinned, he became mortal. That is why man needs to be born again. Our spirits need to go from mortal to immortal. The only way Satan would only be destroyed would be if when he sinned, he also became mortal. But even though the lake of fire burns forever, it also says that the "torment" will also go on forever.
and the Devil, who is leading them astray, was cast into the lake of fire and brimstone, where are the beast and the false prophet, and they shall be tormented day and night-to the ages of the ages. (Rev.20:10, YLT)
And the Adversary who is deceiving them was cast into the lake of fire and sulphur where the wild beast and where the false prophet are also. And they shall be tormented day and night for the eons of the eons. (Rev.20:10, Concordant Literal New Testament, 1983)
...and the Adversary that had been deceiving them was cast into the lake of fire and brimstone, where [were] both the wild-beast and the false-prophet; and they shall be tormented day and night unto the ages of ages. (Rev.20:10, Rotherham Emphasized Bible, 1959
American Standard Version footnote: *Gr. unto the ages of the ages.
Revised Version, 1881 footnote: *Gr. unto the ages of the ages.
Does ages of the ages have an end? Christ's reign is "to the ages of the ages":
And the seventh messenger did sound, and there came great voices in the heaven, saying, 'The kingdoms of the world did become those of our Lord and of His Christ, and he shall reign to the ages of the ages!' (Rev.11:15, YLT)
But His reign is "until" He gives up the Kingdom to the Father:
24 Then the end will come, when He hands over the kingdom to God the Father after He has destroyed all dominion, authority, and power. 25 For He must reign until He has put all His enemies under His feet. (1 Corinthians 15)
So Christ's reign "to the ages of the ages" is not "forever and ever". Therefore the phrase "to the ages of the ages" can be understood of a limited time period that comes to an end. So those in the lake of fire are not punished there "for ever and ever" (Rev.20:10).
Also, "forever and ever" is nonsense. No time can be added to "forever".
When Christ's reign ends (1 Cor.15 above), this will lead to God being "All in all" (v.28). IOW everyone will be saved, as all will be "in Christ" (v.22).
Rev.21:5 He who was seated on the throne said, “I am making all new!”
2 Cor.5:17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all is become new.
Rev.5:13 And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, Blessing, and honour, and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever.
Rev.15:4 Who shall not fear thee, O Lord, and glorify thy name? for thou only art holy: for all nations shall come and worship before thee; for thy judgments are made manifest.
This sounds like just payback, not endless annihilation or tortures:
Rev.18:6 Reward her even as she rewarded you, and double unto her double according to her works: in the cup which she hath filled fill to her double.
Rev.21:5 He who was seated on the throne said, “I am making all new!”
This includes everyone in the universe, including the dead and demons:
Rev.5:13 And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, Blessing, and honour,
and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever.
John speaks of "every creature" & to emphasize this again he repeats "and all that are in them":
Rev.5:13 And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, Blessing, and honour,
and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever.
This worship (v.13) uses the same worshipful words as the redeemed of vs 9-10 use in v.12:
12 Saying with a loud voice, Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honour, and glory, and blessing.
All this being in the context of salvation - "the Lamb that was slain" (v.12 & 13).