Once we reach technological brick wall (10nm), the network hash rate will level off and difficulty might actually stay flat or go down.
At that level, mining=electricity & hosting costs + a tiny profit.
The economic brick wall is 28nm. 20nm is beyond rediculously expensive for anyone who doesn't know their own fab (Intel, Samsung). Nobody is predicting high availability or decent prices of 20nm wagers from any foundry in 2014 either. Nobody (not even Intel) is using 14nm yet, the technical challenges of 10nm haven't even been addresses. Still even if 20/14/10nm is techincally available what matters in mining is the cost per hash and Bitcoin mining is almost perfectly parallel. This means a 20nm chip is going to cost MORE than a 28nm one (everything else being the same) until the cost per transistor is lower on 20nm then 28nm. Usually that takes 2-4 years from process start and it is taking longer and the end benefits are getting smaller (higher NRE, more delays, more complexity, more yield issues, and overall a shrinking gain on the prior process node). So 20nm SHA-2 ASICs probably won't make sense even in 2015 or 2016 and 14nm isn't even on the map yet (2020?+).
That means for the convieable future 28nm is as good as it gets. There may be more efficient 28nm designs but that would mean a brand new NRE cost and it would have to be a lot better to make sense (i.e. a company's new chip would need to be so much cheaper (MH/$ and MH/W) that even after a share of the NRE it added it is still a better deal that then existing design). It remains to be seen if that will happen. Regardless once everyone is on 28nm and shipping in volume, the market will become highly saturated and mining margins will be a small percentage over electrical cost of the most efficient miners. Have a less efficient miner, have higher than normal electrical costs, paying for expensive datacenter space expect it to only be a question of how long before your net operating margin goes negative. Really no different than GPU mining. Those with excessive electrical costs or poorly though out rigs could only profit marginally when the price/difficulty was high and when it tanked they had to idle or mine at a loss.
http://www.electronicsweekly.com/mannerisms/manufacturing/apple-signs-tsmc-for-16nm-and-10nm-nodes-2013-06/
http://www.eetimes.com/document.asp?doc_id=1264668