Thanks to David Larkin for pointing me to this paragraph written by John Stott (David Edwards & John Stott, Essentials: A Liberal-Evangelical Dialogue, London: Hodder & Stoughton, 1988, 315-316, emphasis mine)
The vocabulary of ‘destruction’ is often used in relation to the final state of perdition. The commonest Greek words are the verb apollumi (to destroy) and the noun apoleia (destruction). When the verb is active and transitive, ‘destroy’ means ‘kill’, as when Herod wanted to murder the baby Jesus and the Jewish leaders later plotted to have him executed (Matthew 2:13; 12:14; 27:4). Then Jesus himself told us not to be afraid of those who kill the body and cannot kill the soul. ‘Rather,’ he continued, ‘be afraid of the One [God] who can destroy both soul and body in hell’ (Matthew 10:28; d. James 4:12). If to kill is to deprive the body of life, hell would seem to be the deprivation of both physical and spiritual life, that is, an extinction of being. When the verb is in the middle, and intransitive, it means to be destroyed and so to ‘perish’, whether physically of hunger or snakebite (Luke 15:17; 1 Corinthians 10:9) or eternally in hell (e.g. John 3:16; 10:28; 17:12; Romans 2:12; 1 Corinthians 15:18; 2 Peter 3:9). If believers are hoi sozomenoi (those who are being saved), unbelievers are hoi apollumenoi (those who are perishing). The phrase occurs in 1 Corinthians 1:18, 2 Corinthians 2:15; 4:3, and in 2 Thessalonians 2:10. Jesus is also recorded in the Sermon on the Mount as contrasting the ‘narrow … road that leads to life’ with the ‘broad … road that leads to destruction’ (Matthew 7:13; d. also Romans 9:22; Philippians 1:28; 3:19; Hebrews 10:39; 2 Peter 3:7; Revelation 17:8,11; the word used in 1 Thessalonians 5:3 and 2 Thessalonians 1:9 is olethros, which also means ‘ruin’ or ‘destruction’). It would seem strange, therefore, if people who are said to suffer destruction are in fact not destroyed; and, as you put it, it is ‘difficult to imagine a perpetually inconclusive process of perishing’. It cannot, I think, be replied that it is impossible to destroy human beings because they are immortal, for the immortality-and therefore indestructibility -of the soul is a Greek not a biblical concept. According to Scripture only God possesses immortality in himself (1 Timothy 1:17; 6:16); he reveals and gives it to us through the gospel (2 Timothy 1:10). And by the way, ‘annihilation’ is not quite the same as ‘conditional immortality’. According to the latter, nobody survives death except those to whom God gives life (they are therefore immortal by grace, not by nature), whereas according to the former, everybody survives death and will even be resurrected, but the impenitent will finally be destroyed.