I have provided several examples from God's Word that decisively refute your claim. Before you pontificate like this, you need to address those examples. Start with my case from 1 Corinthians 15:28-29, 1 Peter 3:19; 4:6, and Revelation 21:25. I mean, actually read what I say and don't just jump to texts that you wrongly imagine support your dogmatic agenda!
I don't jump to anything! I do a lot of study on all Biblical topics and I don't say anything unless I know the truth of the matter. If people had the opportunity for salvation after death, why bother receiving Christ in their life time? They could just live according to the sinful nature and then be saved after death. You don't know what you are talking about! Regarding this issue, we have an example of someone (the rich man) who died and went to Sheol/Hades who was in torment in flame. Neither Lazarus, nor anyone else could come back from the dead to warn his five brothers so that they wouldn't come to that place of torment. And he is still there till this very day, where at the great white throne judgment he will be resurrected and will stand before God and be judged and then cast into the lake of fire. There is no offer of salvation after death. Below are the answers to the scriptures you provided:
I Corinthians 15:28
"And when all things have been subjected to Him, then the Son Himself will be made subject to Him who put all things under Him, so that God may be all in all. If these things are not so, what will those do who are baptized for the dead? If the dead are not raised at all, why are people baptized for them?"
First of all, living people being baptized for those who have died is not supported by scripture. Do you also believe that women will be saved through bearing children? Because Paul said that as well and we know that salvation cannot be obtained by getting pregnant and having children. Therefore, in saying this Paul had a different meaning in mind. He could have been speaking about the existing false religious beliefs where they believed in being baptized for the dead, where Paul could have used it as an example. It was because the Corinthians were not believing in the resurrection that he mentions baptism. Living believers in Christ being baptized for the unbelieving dead would go against the rest of the scriptures. The phrase "Today is the day of salvation," means to seek the Lord while you're living, during the time of God's favor. For once a person dies in their sins, God's love, grace and forgiveness are no longer in operation for that person. They will be held accountable for every idle word at the great white throne judgment.
But back to the issue: baptism is a public proclamation of ones true repentance. It signifies the believer dying to the old nature and rising in the new man in Christ, being buried with Christ and rising to new life. A person who has died without believing cannot make that proclamation. They died in their sins and will be held accountable for their sins, every last one of them. For regarding those who die in their sins Jesus said the following:
"But I tell you that men will give an account on the day of judgment for every careless word they have spoken." (Matt.12:36)
"Anyone whose name was not found written in the book of life was thrown into the lake of fire." - (Rev.20:15)
There is no offer of salvation after death and the judgment is final and eternal, everlasting. For they will have died without the blood of Christ covering them and there is no other way of being saved. Therefore, after the unbeliever dies they have no opportunity to be reconciled to God, which can only be done by the shedding of righteous blood.
I Peter 3:18-20
"For Christ also suffered for sins once for all, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the spirit, in which He also went and preached to the spirits in prison who disobeyed long ago when God waited patiently in the days of Noah, while the ark was being built."
First of all, Jesus did not go and preach to the unbelievers in Sheol/Hades to offer them salvation. What I believe this scripture is saying is that, Christ went to the spirits/fallen angels who attempted to corrupt the Messianic blood line when they took wives from the progeny of men and produced children/giants. He went to the place where they had been sent (Tartartus/Tartaroo) to announce to them that they didn't succeed in their attempt and that He had redeemed mankind, all those who would believe in Him.
"For if God did not spare the angels when they sinned, but cast them deep into hell (Tartartus) placing them in chains of darkness to be held for judgment" - (2 Pet.2:4)
"And the angels who did not stay within their own domain but abandoned their proper dwelling—these He has kept in eternal chains under darkness, bound for judgment on that great day." - (Jude 6)
In order to come to a conclusion regarding any Biblical topic, we must consider all related scriptures and not just depend on one. Click on the link below for information regarding the possible means of what Paul meant regarding those being baptized for the dead. Please don't just cast it aside, but read what it says if you want more information on it.
https://www.gotquestions.org/baptism-dead.html