The issue revolves around the word forever, as I see it. The reason I came to believe in the salvation of all was that after a brother presented it to me in brief, over the next ten years or so I began to see some very clear scriptures about the reconciliation of all, the restoration of all things, the gathering together of all things in Christ.
I am not a literalist in a strict way, but I strongly believed in the overall integrity of the scriptures so I had to ask myself, “Where is the seam between these views?” That is when I began to look into translation issues, because I believed then that both(or all three if you like) cannot be true.
One of the main things that spoke to me was that if every adversary is subjected and death is anulled and God becomes all in all(1 Cor 15), then only annihilation or UR could be true because if death is anulled the second death cannot last forever.
But if annihilation is true then God would not be all in all (1 Cor 15:28). All things could not be reconciled to God by the blood of His cross(Col 1:15-20). All things could not be gathered into one in Christ(Eph 1:9-11). Unless perhaps one wanted to add “all that is left”- but I do not believe the scriptures supply that interpretation in the verses in the original languages.
So I began to study “olam” and “aion” and came to believe that “eternal” is not a state of endles time, but like “olam”, the indefinite time according to the subject to which it is applied(450 Old Testament occurences) and that aion, in a way similar to “olam” was a definition of periods(ages) in the plan of God, seen through a glass darkly.
Now, I see so many verses in the OT and the New that declare(imo) that God will reconcile all things and restore all things because Jesus has been exalted above all things, and every enemy will be made a footstool ofr His feet, and every knee shall bow, whether in heaven or on earth or under the earth, and every tongue confess that Jesus Christ is Lord to the glory of God the Father. When the last knee bows, death will be anulled along with “all rule, power and authority”, because God will be "“everthing in everyone” and love will rule the whole creation. Rule, power, authority and death will no longer be needed, because sin will be vanquished in its entirety. For punishment to last forever, sin must abide forever(imo).
For me, it has become quite explicit, because it is the only way thatI can see for all those scriptures can be integrated and find harmony.