I believe this is a misconception. It is based on the idea that the the goal of the Buddha’s path is something that is only fully realized at the point of the arahant’s death. But the goal of the Buddha’s path is attained during life. It is the unsurpassed bliss, peace and freedom that are experienced when the asavas are totally destroyed, the burden of attachments is fully released, the I-making and my-making processes are temporarily halted and dukkha is brought completely to an end.
K. R. Norman has a paper in which he clarifies the distinction between nibbana and parinibbana. This is not distinction between nibbana during life and nibbana in death. Similarly it is not a distinction between a “preliminary” nibbana and “final” nibbana. According to Norman, the prefix “pari” is just used to signify the difference between a state and the event of the attainment of that state. Nibbana is the state of being released, and a parinibbana is the attainment or achievement of nibbana.
The Mahaparinibbana sutta is not a sutta which describes how the Buddha, at long last, achieved some exceptional condition of “parinibbana” which can only occur at death. It is a sutta which describes the last time the Buddha attained nibbana, something he had done many times before. It is a “great” sutta because it is very long, and describes events of great significance.
Perhaps one problem in contemporary treatments of nibbana is that they are strongly inflected by the western Protestant conception of the “assurance of salvation”. There are some versions of this notion in some of the early texts as well. The idea here seems to be that the real or total nibbana is both the ultimate goal and something that only happens when the arahant dies. So that then raises the question about what the heck happened under that Bodhi tree. The answer that is sometimes given is that the main point of what happened under the Bodhi tree is that that’s when the Buddha realized that he had finally brought an end to the kammic processes that produce rebirth, and so his happiness consisted mainly in the assurance of the fact that he was bearing his last body. Salvation is construed purely negatively as the ending of everything, and the greatest happiness that occurs during life is nothing but the pessimist’s relief over the fact that he has finally succeeded in bringing it all to an and, and now all he has to do is wait patiently for his long sought-after extinguishment. Apparently he even wants happiness to be extinguished.
This misconception, if I am right, risks turning “Buddhism” into little more than a miserable but pious suicide cult in which people are terrified by the endless stretches of sadness they imagine before them, and desperately try to end it all. Ending it all is itself mistakenly viewed as the goal. They only reason such people don’t “use the knife” is because they think they are trapped and will only be reborn again. Practicing this form of Buddhism actually risks making people more unhappy and depressed.
This completely misses the happiness the Buddha taught people how to achieve, and that is frequently attested in the joyful verses and other recorded words of the arahants. The correct conception of the goal is a much more optimistic picture, because even if one doesn’t attain the summum bonum, the supreme bliss of total release, the path through lesser attainments is an assent through higher and higher forms of happiness. So the path is eminently worth pursuing even if the goal isn’t achieved.