Bit depth is a fine term, but steps is not. If there are steps, the signal has not been properly digitized or generated. If you see steps in a gradient, there was insufficient noise in its capture or generation. See, for example, Quantel's Dynamic Rounding or, better yet, the lectures and courses of Stanley Lipshitz at the University of Waterloo (if you EVER get a chance to attend one of his digitization demos, drop everything and do so; they are exceptionally enlightening). Again, all more bit depth buys you is more SNR (not an insignificant characteristic).
As for human vision, I'm afraid there's a lot more going on than iris. If you spend time on the beach and then head indoors, you'll miss much when you first enter, despite your pupil's growing. After a while, however, you'll see fine. The same is true when you leave a movie theater at noon. Your iris shrinks, but you initially see everything all washed out. There is chemical adaptation taking place, and it takes time. There is also contrast compression taking place in the retina before signals are sent to the visual cortex.
Having dabbled in contrast compression intended to emulate human vision (and having built successful prototypes), I'll just say that maybe you don't really want to match human vision (that was our conclusion).
You are correct that the dynamic range of some imagers is increasing; that of others, unfortunately, is shrinking with the smaller imager size and increased resolution (both create smaller individual sensors). I suspect that the video camera with the greatest dynamic range is probably currently the NAC Memrecam fx K4, rated at ISO 10,000, but there are many reasons why you might want to use something else.
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