A lot of us have been talking about ETH difficulty and estimating the Ice Age date. EIP #649 has been implemented for the Metropolis release. I expect that within the next 4 weeks. The summary specifically states they aim to get the average block time back below 15 seconds. It's now over 21 seconds. V.Buterin proposed the delay by 42 million seconds (or 1.4 years) so that it would trend to 30 second block times by the end of 2018.
Part of the initial proposal also included a reduction in block rewards from 5 per block to 3. This was also a direct proposal by V.Buterin, who thinks even 2.5 ETH per block is OK. It looks to me like this issuance part of this EIP was removed. H
The bottom line, as I see it, is this:
1)They need more time for POS (Serenity). They are not doing this for the benefit of miners. The developers do not seem to care in the least bit for miners. You need to keep that in mind at all times. To them, miners are purely profit motivated. They base their decisions off of that, and they are probably right for the most part.
2) The potential lifespan of mining will be extended until the end of 2018. This depends on the price of ETH and your specific cost structure. I think it still may be shorter. I don't think they liked being under the gun so much when the Ice Age was ramping up so quickly. They may just be giving themselves far longer than anticipate given that they have been behind schedule. End of 2018 should be the maximum of anything you consider.
3) You should not be building new miners. Quit guessing on paybacks. End of 2018 is looking like a pretty firm date, at the latest. You are probably not going to see a positive ROI on any new rigs over this period. Keep running what you have and sell your gear before this curve comes up next year. That is, as long as there are no alternates by then. I'm not even sure that really matters as the flood of miners at the switch will immediately impact any viable alternative.
4) ETH technology keeps getting better. This bodes well for the value of ETH. If you think ETH is going to increase in value, you will make far more money buying ETH directly than putting those funds into a miner.
5) Don't build new miners! Yeah, I said it twice. It doesn't make sense. Each day new people are coming in without doing a bit of research. You can see that by the simplicity of their technical and other questions. If you read this and decide to ignore it, thae it's your own fault. Buy ETH, support ETH, but economics of mining are over.