Not just to prolong battery life, to prevent the device from overdrawing the battery and shutting down when it couldn't provide the same peak current as it could when new.
This was likely a compromise from that design decision.
Since the slowdown was based on battery health and not absolute age of the device it was (and is) still possible to replace the battery.
But “the market” (ugh) seems to have decided that it wants phones with integrated batteries, and touch screens, and no physical keyboard, that are too big to use with one hand.
