How would non-permanent credits even work? You purchase the carbon-offset option on a flight, a tree gets planted somewhere, ten years later it burns in a wildfire. Then what?
This also addresses the tree death issue.
Pyrophilic species can even benefit from forest fires with reproduction being triggered by them.
It would also be quite unusual for accountant teams to follow individual trees around throughout their lifecycle, ticking little boxes when they die or something, in order to follow through on carbon credits. Think about this realistically.
There's no guarantee that a tree planted to "offset" a flight will grow to any kind of maturity, let alone survive three months. Some saplings just die.
Or maybe that tree got planted in a tree plantation for timber, or wood pulp, and 20 years later, it's framing timber or newsprint.
(I am someone who does revegetation projects and have planted a large number of trees)