If there is no rebirth than the path is mostly pointless. It doesn't matter if one is serial rapist or a saint, it doesn't matter if one has wrong or right views, it doesn't matter if one is Buddhist or fundamentalist radical Muslim, the end would be the same and suicide could simply hasten nibbana.
Also even if one were to suffer continuously for 120 years, the cumulative suffering would not be even 1/trillionth of the cumulative suffering possible due to trillions or more of rebirths.
By denying the full amount of suffering, one is denying 1st NT. If the suffering is believed to be not that much (even 120 years of suffering is nothing compared to cumulative amount of suffering in rebirths), then it doesn't really make much sense and urgency to practice the path.
Why do we need the Buddha if we are guaranteed parinibbāna at death?
"Life is a struggle. Life will throw curveballs at you, it will humble you, it will attempt to break you down. And just when you think things are starting to look up, life will smack you back down with ruthless indifference..."