Mind you, the A14, A15 and A16 all have as powerful of a Neural Engine as the M1 has. So either it isn't related to the SoC at all, but solely to the RAM (in which case Apple is an absolute idiot for going so long with so little RAM in such expensive devices, which incidentally is something people have warned them about that the amount of RAM they gave their devices isn't very future-proof) or this very much is just to get people to buy the next iPhone or the current but more expensive iPhone Pro.
Either way, the sudden explosion of AI absolutely caught Apple with its pants down. The M4 is just another example of that.