Reason #2 - I'm not going back.
Perseverance and assurance may go hand in hand, but they're not the same thing.
Perseverance and "once saved always saved" may sound like they're the same, but they're not.
Assurance is in the "now" - right now, you can be sure of your salvation.
I write these things to you who believe in the name of the Son of God that you may know that you have eternal life - 1 John 5:13
"Assurance" means that right now, this very minute, you can know that you have eternal life. It may be, if you believe such things, that tomorrow or the next day...or sometime in the future...you may sin too much or do too little and lose it. You can be assured now...but tomorrow may be a different story. You have the choice, in the future, to walk away.
"Once saved, always saved" gives the impression that..well, just what it says. No matter what you do or believe, you have no choice. It's the "fire insurance" way of thinking. Even if you want to walk away...shoot...even if you DO walk away...you're stuck with your salvation. "Once saved, always saved" is about what you have.
"Perseverance of the saints" is very different. It's not our salvation that is preserved...it's God who makes us persevere. If we are in Christ, we don't WANT to walk away.
"6 And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ" ` Philippians 1:6
We are now, and will be, assured of our salvation because we are preserved and sealed by the Holy Spirit.
In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will, so that we who were the first to hope in Christ might be to the praise of his glory. In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of his glory ` Eph. 1:11-14
Conclusion - both "perseverance" and "once saved, always saved" comes with assurance, and even "I can lose it later" can have assurance in the present.
"OSAS" - you don't have a choice if you wanted it
"POTS" - even if you have the choice, you are sealed by the promise that you won't want the choice.