Eternal life is eternal, because it is “in Jesus”. Eternal life will never cease, but those who will not abide in the vine, cut themselves off from their share of eternal life. The gift is not independent, it is dependent and ongoing John 15
If someone has “eternal” life and then quits believing and ends up in hell then the life that person received and had for a season will not be eternal for them. The seed was not imperishable in their case. The Holy Spirit was not a guarantee of their inheritance, there was no guarantee in fact. They were saved for a season until God reversed His decision to save them. God was not able (or willing) to keep them from falling.
I understand your concern about Christians believing in easy grace and saying a sinners prayer but not truly repenting. I agree that we must persevere until the end, and that without holiness, no one will see the Lord. We must work out our salvation with fear and trembling. We must make out calling and election sure. If someone walks away from Christianity and never returns, then I would say that person was never saved. What does Jesus say to the goats on judgement day? Does He say “I used to know you until you walked away”? Or does He say “I never knew you”?
The warnings against apostasy are there for a reason. They encourage us to persevere and to fear God. However, I believe that we can be confident that the same God who drew us to Christ and removed our hostility towards Him, will be faithful to complete that which He started. I believe He is both able (and willing) to keep us from falling. I believe that nothing can separate us from His love (nothing includes our own tendency to grow cold in the faith). Whoever is born or God does not go on sinning (we all sin, but we do not remain in a lifestyle of unrepentant sinning). If a believer sins they will be disciplined and if a believer commits a serious sin the discipline will be severe. God will not cast us out though once we are brought into His family.
The word “eternal” has scared me since I was a child.