Planned obsolescence is usually taken as a "bad thing" but it isn't necessarily - when considered as "there's no reason to spend money on making a part of this outlast the useful lifespan of the whole."
A cruise missile doesn't need to fix a bug that can only occur months after it has detonated, after all.
The real problem comes from the manufacturer's idea of "useful lifespan" and the customer's. For most people, an appliance's useful lifespan is "as long as it keeps working" and they don't care to ever upgrade it if it doesn't fail. Manufacturer's aren't incentivized to make those, however, and only accidentally do so (something designed to last 10 years is likely to last much longer on average).
I think what you call the 'real problem' is what people mean by planned obsolescence. It does not refer to any sort of tradeoffs, its specifically used to refer to times when the manufacturer sets their useful lifespan lower than the customer would want, and lower than the optimal price/lifespan ratio, so as to force more sales
Usually you can find the price/lifespan ratio you want - by paying more.
What the customer wants is the cheapest item to last forever, but also be cheaper. There's obvious tradeoffs here, and the problem for everyone is the customers keep buying the cheapest ones.
"Limited lifespan is only a sign of planned obsolescence if the limit is made artificially short."
Its not always the consumers fault, its a market for lemons and paying more is by no means a guarantee of a better product. Especially when models are updated every few years , so keeping track becomes almost impossible.
Most examples have some sort of devils advocate reason (which I rarely believe) but some are extremely cut and dry:
"For example, inkjet printer manufacturers employ smart chips in their ink cartridges to prevent them from being used after a certain threshold (number of pages, time, etc.), even though the cartridge may still contain usable ink or could be refilled (with ink toners, up to 50 percent of the toner cartridge is often still full)"
A cruise missile doesn't need to fix a bug that can only occur months after it has detonated, after all.
The real problem comes from the manufacturer's idea of "useful lifespan" and the customer's. For most people, an appliance's useful lifespan is "as long as it keeps working" and they don't care to ever upgrade it if it doesn't fail. Manufacturer's aren't incentivized to make those, however, and only accidentally do so (something designed to last 10 years is likely to last much longer on average).