Women gestate for varying lengths of time--40 weeks strictly represents an average and nothing more. 40 wks takes into account that most normally healthy women, with normally functioning placentas, give birth normally to sufficiently developed-and-ready babies, most often between 38-42 wks...which mathematically is a 40wk average (stating this very simplistically). Now, there are occasionally underlying physical or physiological issues for mom and/or baby which prevent labor from starting in that time frame--but most pregnancies that go to 42 weeks or even a week or more beyond that, are still normal. Placenta still functioning and all!
"Placenta failure" is just another 'dead baby card' that is pulled by HCPs who do not fully trust or understand birth, and who prefer to control birth as much as possible (whether out of fear, or out of convenience or both). And of course, it is far easier to control moms who fear for their baby's life, than those who feel that they and their baby are perfectly fine.
Placentas are amazingly complex and highly ABLE organs that perform a multitude of tasks. Even a placenta that has indeed 'begun to expire' can fully support a healthy baby for awhile. Even a placenta that is partially abrupted can fully support a healthy baby for some time.
There is even some thought that it is a normal element of the complex processes which initiate normal labor, for the placenta to start 'failing' That is, that the initial stages of a placenta's 'expiration' sends chemical signals to mom and baby both; this is part of an intricate hormonal/biochemical dance between mom and baby, a cascade of biochemical events that leads to labor and birth, bonding and lactation, etc. And in most cases, a placenta that has 'reduced functioning' in late pregnancy is still functioning plenty well enough. Which only makes sense since pregnancy and birth are so very well designed, and most often work so very very well.
It can truthfully be stated that a placenta has 'an expiration date'--but it is not truthful to say that that date is at 40wks for any/all women. A baby who is growing well and is still adequately vigorous inside a mom who is still normally healthy no matter how many weeks gestation, still has a fully-enough functioning placenta. The very most usual 'full expiration date' of a placenta turns out to be the day of birth, whether earlier or later than that 'average'.