> Batteries have been used as part of planned obsol[esc]ence for too long and a whole small business industry of replacing phone batteries has appeared because of it.

Note that early phones had replaceable batteries and it was later phones that dropped that feature. The idea wasn't that making the phone impossible to open would compel people to replace their phone faster; it was that given that people didn't keep their phones long enough to wear out the battery, there was no need to make the battery accessible.

That was true 15-20 years ago. Nowadays changing the phone is basically because:

1) battery dying / not lasting enough

2) shattered glasses whose replacement costs 35-40% of the cost of the phone new (for budget/mid-range phones, not everybody has iPhones)

distant 3rd) not enough free internal storage

Upgrade cycles have slowed down in recent years, the improvements are relatively incremental nowadays. Screens, durability, processors, storage sizes, cameras, even battery life are okay-ish and aren't improving quickly enough to justify the same upgrade rate. Foldables are basically the only big innovation in recent years, but are still a little too fragile and expensive.

This is also reflected in the increasing support durations from major manufacturers.

This might be partially true, but making them inacessible is still a great way approach to planned obsolescence and there's no way this was not part of the motivation. The fact that an entire industry exists to provide replacement batteries is proof of this, as is the fact that Apple offers a £100 battery replacement. They also replace the batteries of all refurbished models they sell, which again wouldn't be necessary if battery life wasn't a concern over the useful life of a phone.

Secondly, what you said may have been true in the past, when smartphones were rapidly evolving and upgrade cycles were short, but people are holding on to their devices for longer now, so its possible its becoming a problem again.

Batteries on early cell phones needed to be replaced multiple times a day. I remember talk time of like 10 minutes on my motorola StarTec.

Nowadays batteries seem to be doing pretty good, though. I've got a galax s20 fe, and the battery is still fine after 5 years.

This was true back when Moore's law was the driver of obsolescence. You bought a new phone every year simply because next year's phone was twice as fast.

Now that this doesn't happen, the driver of obsolescence is the battery, which is much less defensible because you can swap it much more easily than "the whole internals of the phone".