What is written, is written: the Gospel was preached to the dead, and it is a forceful twisting of text to interpret it in the way that Peter referred to people who were alive when Gospel was preached to them, but have died for the moment when Peter speaks about them. Rather, Peter says that Gospel was preached to those who already have been dead at the moment of preaching to them.
This means that even after physical death they retain their living personalities who can understand the Gospel and be gladdened by it, for εὐανγέλιον means "gladdening news". Those living personalities who can understand and be gladdened by that what they understand can also be called traditionally "souls", or "spirits" (as Peter does in 1 Peter 3:19), both words being metaphoric words for indicating the invisible and dynamic in human essence.
But who preached to them? Apostles could not go to the threshold of Hades like Ulysses and summon them from there, to be sure, but Jesus Himself, who is Lord over the living and dead (Romans 14:9), went to them and preached to them the good news about abolition of the reign of sin and death (cf. 1 Cor. 15:55-57).
There is a heresy that crept in Christian thought already in early times that souls co-die with bodies, which heresy was called "thnetophsychism" ("soul-dying") (2nd-3rd century), but correct vision is that human persons' invisible core does not co-die with body but survives body and is transformed in a different dimension, with different conditions, better or worse. That's why to pray for the amelioration of the condition of the dead loved ones is an indispensable aspect of mainstream Christian traditions, like Catholics and Orthodox (I do not know how exactly in the versatile spectrum of Protestant tradition, but there also), who include in their liturgical course the prayers for the dead, and also prayers to the dead saints who are in a position of boldness before God so as to be able to help us.
How otherwise?! Jesus is Lord and God of living and dead simultaneously (1 Cor. 15:55-57). Now, what is more reasonable to believe, a) Jesus is actual Lord of only a minority of humans, who live at any given moment of history, while only prospectively, after their resurrection at the end of history, the Lord of also the majority, who are no more alive physically at a given moment of history; or b) Jesus is actual Lord of all, both the minority of now living humans and majority of the passed away, and all are alive before Him, for He is God of living, not dead, for all (the deceased Abraham and Isaac etc. included) are living for Him (Mark 12:27)?
Surely, the second is more reasonable and biblical, for if the first is correct, then we get a monstrosity: God remembers a dead person, who no more exists, and at the final resurrection will not resurrect the guy, but in fact, re-create him according to His memory. But then why should He re-create sinners? For what is sin? Sin is a damage of inner self, so we shall get God who creates a damaged inner self, a damaged and sinful person, which is a plain calumny on God and as such a blasphemy.
But let us return to sound theology and affirm the traditional theological position of immortality (not natural but by grace of God) of souls, which not only Christians but even Greek philosophers realised by following dialectics. In fact, death is always, necessarily, an outcome of a damage, to the effect that what is not damaged cannot die in principle. Now, when body dies, we understand that it is damaged and that's why it ceases to function, and we can describe this damage. But the most important aspect of our essence, the moral integrity, the love of truth, sense of justice, admiration of God - all those invisible and most important features that make our inner self, are they destroyed when body is killed? Impossible, because oftentimes in history humans choose rather to die physically, but keep those features intact and healthy; that is to say, they even rescue the health of those features at expense of physical life. Thus, those features cannot die for they are not damaged, moreover, they are even protected and rescued at expense of the physical life, for a person who gave life for them, those features, deemed life and welfare of those features higher than his physical life.
Now, is not it utterly unreasonable to think that while those features that truly embellished this person are rescued, the person who has rescued them in himself is annihilated and ceases to exist? For if the foundation of those features' sustenance which is the person's will is annihilated, then those features will also be annihilated; but given that they are not, due to the fact that their damage was prevented by, say, a heroic death, then we must necessarily conclude that the decision-maker in him, the rescuer of those divine features in himself, also survived the physical death. Those are not my arguments, but I made a freestyle elaboration of core argument of Plato in his "Republic".
What do the great Christian saints and martyrs like St Ignatius of Antioch thrown to lions in Colosseum, or St Sebastian, or St Justin the Martyr think when they see that some of their misguided co-christians do not believe that they at this very moment stand before God and pray to Him and can aid their brethren and sistern who are now historically living? Do they smile, or are angry, or filled with a pity to the point of tears?