Even if a 1TH/s miner was produced every minute the difficulty would only increase about 212M per day. If we assume an exponential growth rate of 20% per difficulty change, the increase in hash rate would exceed the production rate of miners in early June. The growth in difficulty will become linear.
Why would it be linear? I don't get it. The production capacity of 28nm (or 14nm) products can be increasing at exponential rate. As long as human population increases at exponential (or near exponential) rate why would production (of asics or anything else) not be in step. It has been so far.
We still have enough resources on this rock to have a good exponential run for another 75-100 years or so.
There will be nothing linear about it in our lifetimes. Babies are born every minute and every minute there is a teenager somewhere who discovers crypto currencies and wants to make some money.