Somedays I think we're approaching it, others not so much; it honestly depends on what the people will consider good enough for ease-of-use. A phablet is paradoxical, isn't it? Phones getting more smaller and portable for decades, and then they get bigger, and bigger, and bigger - but why?
But it (all overall re
lateau) depends on material sciences, battery tech, and the like. If we're approaching the limits of batteries and materials, then of course, we're closer to plateauing, the next step is dispersing the tech everywhere. If we're close to a breakthrough (carbon materials? diamondiod? Fusion batteries?) then it'll keep on going.
But it (all overall re
