In reverse, there has never been a way to upgrade, I.e., increase, the memory in an iOS device. When the ram is integrated in the CPU chip and not separate, there is not a way to increase the ram without building a new logic board.
It is not unusual for operating systems to grow and leave the older hardware behind. Adding more capability places more demand on the hardware that may not be able to be met while maintaining acceptable performance. That has always been a fact of life in the world of computers...from vacuum tubes to transistors to solid state, integrated devices.
iOS began as a dumbed-down Mac OS X for personal devices that lacked the power of the personal computer.