A certain amount of planned obsolescence can be a force for good. It makes sure that production capacity does not become stagnant and disappear. But senseless cost reduction is an atrociously bad thing.

I draw a hard moral line here: Intentionally designing failure into a product is just plain wrong.

I don't mean fail safes, like building a table saw that fires an explosive charge to stop a spinning blade when it detects the presence of a finger [0]. If some given production capacity can only be sustained by selling fresh barrels of oil converted into easily-broken home goods is it worth sustaining?

[0] https://sawstop.com

On the other hand, perishable stuff is pretty natural. If anything, it is non-perishable stuff that seems odd... There's nothing eternal in an ecosystem. Individual animals and plants die continuously and their stuff is recycled on an on. The individual cells on your body die and are replaced.

While I absolutely abhor that you cannot buy a toaster that lasts more than two years, I still see a case for well-thought-out obsolescence to be an important ingredient in sustainability.

Think about a spaceship that is moving a city to another star system for a multi-generation trip. Would you embark anything that is non-perishable? What if it breaks by accident? It is maybe better to make sure that nothing lasts more than a generation, so that there's always people having the experience to build it again (because they have already built it before).

I'm not convinced by your "moral hard line" against planned obsolescence. It is a matter of quantity: today our technology is sadly too perishable. But a (tiny) amount of planned obsolescence may still be a good thing. Just don't say it too loud :)

Mechanical design is all about designing failures.

You want the thing to break in a safe way where you can tell before hand that it's aging and needs replacement

That’s not designing failure into a product. That’s designing a product so that when it does, it fails in specific ways. The latter is just good product design and engineering; the former is what’s arguably unethical.