1) Should we expect to see faster miners than S9 and other similar? they've been pumping those out from the factory for quite a while now.
2) Would miners put old miners back online at these prices?
1) I think, and correct me if I'm wrong, that the high end of the efficiency curve of the current 16nm form factor is pretty well represented in the current generation of gear. There might be a small incremental upgrade in gear coming in the next few months, but I don't think there will be any exponential increases in efficiency until a new generation of gear is released based on the next form factor, 10nm perhaps. But who knows. 10nm gear suitable for mining is still a long way off, and I mean a long way. At least a year or more. Personally I think we're looking at well into 2019 for anything in that form factor. So what miners have to do to keep up with difficulty right now is continuously add more hash at about the same efficiency. So mining is becoming more and more of a volume endeavour, and this gets a little tricky when you are adding gear that is mature in the current efficiency cycle. Obsolescence in this biz is a hard thing to predict, and yet you still need to try to make reasonable assumptions as to the best way new capital should be deployed in your mining operation.
It's interesting to me that the current situation is forcing miners to add capacity rather than efficiency for such a long time, so when the next generation of gear is released there will be a gigantic amount of worldwide capacity ready and hungry for the upgrade. This means the economics of the next upgrade cycle will be very 'different' from the previous ones, and the experienced miners know exactly what I'm talking about.
For near term incremental upgrades, I'm hoping to see someone break BitMain's monopoly on efficiency. The 741, for example, is a better design, they are really excellent machines (we're adding 12 more next month), but lack the efficiency to compete at the top end with the S9. Especially if you are trying to pack on as much hash as you can with a limit to space and electric capacity, or you are mining with marginally expensive electric rates. I'm hoping Canaan will release a new series of miners in the near future that are of equal quality to the 741 (you know, they don't break) but match or even slightly beat the efficiency of the S9 at a similar cost per TH (the old cost last summer, not the new BCC one please). That would be a short term game changer.
2) When the price of BTC rises faster than difficulty increases can keep up, older units may be put back into production for awhile. Also, if a miner thinks BTC is going to say, 10K near term, he won't care if the older gear mines at a small loss 'right now'. If I had the capacity at the moment I'd throw the 20 or so recently retired S7's I have laying around here back up on the rack and run them. I ended up running those machine a LOT longer than I ever thought I would due to the steady rise in BTC price, and we still throw one in an S9 slot from time to time when one comes down for maint. Notice I didn't say 741 slot, because those never seem to need to come down for maint.