In reality, he could have chosen a smoother approximation of Wt[t] – reducing the block reward every 10 blocks, or every 100 blocks, or every 1000 blocks.
Of course! But I also think Bitcoin wouldn't have been quite as successful with a smooth emission curve.
I think he opted for a halving of the block reward every 4 years simply to make it easier for regular people unfamiliar with mathematics to remember and comprehend.
It's a great hook considering how weird and confusing the other mechanics of Bitcoin may seem at a first glance. All things considered the halving is one of the easiest concepts to explain to a newcomer.
Apart from that a smooth emission curve wouldn't have resulted in the sudden supply shocks that lead to Bitcoin's notorious bubbles which in turn placed it on everyone's radar. Add to that a cycle of predictable, publicly known, future supply shocks and you get yourself a self-fulling prophecy -- not only because one "knows" when to expect the next price increase, but also because price increases are expected by most market participants at roughly the same time.
Why was satoshi so obsessed with the number 8? 
The number 21 was just a distraction all along
