It's less about planned obsolescence that's built into multi celled organisms. The choice is how much energy you spend fighting it.

EX: A fruit fly needs far less cancer defenses than a squirrel. And, 2 fruit flies can become 2,000 fruit fly's a lot faster than 2 squirrels can become 2,000 squirrels. But, fruit fly's can't eat nuts so as long as trees produce nuts there is a nitch it takes something with at least the mass of a squirrels to fill.

> but what and why?

Iron and heavy metals accumulate in tissues over time. The tissue membranes accumulate an increasing share of polyunsaturated lipids, which in turn interfere with efficient metabolism. Many ways that biological tissue wears out over time are well understood. It's not just some mystery.

That doesn't really ring true - after all, they're going to be passed to newborns too. "Gunk" collects in tissues that are never replaced, but why does that occur in the first place? Clearly, growing new tissue isn't a biologically hard problem - when there's evolutionary pressure to do so, several species evolved means to replace surprisingly large parts of their body. Some insect metamorphose; crustaceans rebuild complete skeletons - energetically, it's almost certainly hell of a lot cheaper to occasionally replace bad bits than it is to procreate and lose the previous generation. There's something else going on there.