He almost surely meant simplicity of use - of portability. He clearly understood technology advancing, ie, becoming more complex technically - he also realized that it would make using them simpler.
It’s far far far easier to make a call today from almost anywhere...a boat out at sea, in the middle of Africa, or in your home or office....than it was to make a call from your home back in his day....
That's not what he said. His statement was - "the instruments through which we shall be able to do [t]his will be amazingly simple compared with our present telephone".
I suspect he envisioned a system akin to his idea of power transmission to everybody with no moving parts, just a bunch of antennae put up to catch electricity from the air.
“Predicting the future” is simply a matter of observing and understanding the present to the point that “predicting” what comes next is trivial.
There are few humans throughout history with this type of brain/mind construction capable of far exceeding “normal” capacity to observe and understand.