You're missing 2 things.
1) When the S2 was announced, Ant S1s were 0.98
BTC, and the coinbase price was $634. This made the cost of buying 1TH/s of S1s almost $3200. Now you still had to spend a few hundred in PSUs, but you were hashing within a few days. The point is that upgrading only cost an extra $400, and you could cut your power consumption in half.
2) The chips in the S2 aren't "underpowered". The reality is that Bitmain is using the same chips in the S2 as what they've already been using the in S1. The difference is that the S1 chips were set at max voltage and max clocks, for max performance. The S2 chips are set at the lowest voltage, with a much lower clock rate, for max power efficiency.
Here are the chip ranges:
The S1 uses 8 banks of 8 chips (total=64 chips) at 2.8GHs/chip for 179.2GH/s. The S2 probably uses somewhere in the range of 600-650 chips at 1.6GHs/chip for a total of 1TH/s.
It's theoretically possible to take an S2, and increase the chip voltage and clock speeds to higher, but IDK if the boards can handle it, nor if the PSU it comes with can handle it. You could probably OC to 1.2 or more pretty easily, tho.
Well put. The third thing the OP is missing is the cost of PSUs to power those 5 overclocked S1s. S1s do not come with a PSU, S2 does.