The assertion that the network will stop growing after 200-300Ph is a very very bad one. Just because retail customers might not pay $0.50/Gh doesn't mean that manufacturers cannot keep making equipment for their raw costs.
Look at who is mining today and their percentages.
Bitfury - 8-10%
KNCminer - 6%
Antpool - 4%
Cloudhashing / PeerNova - 3%
That is a full 25% of the network from just manufacturers mining. And this is only what we know about.
Do you some how think that when customers stop buying the manufacturers are going to stop producing and mining?
But every hardware manufacturer has some fixed costs which can't go down no matter how cheap is the chip. PSUs, cases, fans, heatsinks etc so the price can't be extremely low. It would be interesting to hear some numbers for the fixed costs per GH/s. I'm thinking that they are around 0.1-0.3$/GH/s for a full system without the chips.