P.S. Planned obsolescence is 99% a myth. People confuse it with “engineering down to a cost”, which has very different motivations, and with design tradeoffs, like choosing thinness and water sealing over a battery compartment. That and people simply not understanding that some things wear out.
Perhaps better said as 99% misunderstood because planned obsolescence is very much real -- a product designer in my office once showed me something he was asked to design such that it have to be mounted on a plastic bracket... his specification stated that the bracket must break after a single usage and the motivation was that the client's customer (a gov't organisation) would have to buy tens of thousands of those things.
No, it's a myth 99% of the time, as I said, and your example is precisely an example of the remaining 1%. (I'd also argue that if it's not
supposed to be a consumable or a sacrificial breakaway part, then it's an example of outright fraud.)
In the vast majority of products, manufacturers want it inexpensive, but not fragile, because they're responsible for the warranty fulfilment. Not to mention that if products consistently lasted only as long as the warranty, the company's reputation would ultimately reflect this. (The fact is, most products still generally outlive their warranties significantly.) If companies tried to make them last, say, 2 years instead of a natural lifespan of 5, then natural variation would mean that they'd be pushing a much larger "tail" of the curve back into the warranty period, which could end up costing them a fortune, not to mention that people burned that way would be much less likely to buy from them again.
Granted, in the smartphone battery example it is probably more like "tacit obsolescence" because Sony didn't see fit to ship the phone with decent protections against over-discharge.
Natural obsolescence is a thing tons of people don't understand. Like… technology progresses, and it's unreasonable to expect it to old stuff.
As for the batteries, given that Sony is one of the pioneers of LiIon, and a giant in that industry, and never a cut-rate company, the chances that they did too little is, well, nonexistent. Chances are, what you're considering the lack of decent protections is in fact
those protections doing their job.