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.