No I do agree! We all want faster and better, but I expect the GPU that I buy to be beaten by the next GPU in that line up. Not by a lower tier lineup.
The reason why the Titan X is being beaten by the Mid range GPU is it shouldn't have been a 28nm fab GPU, it was. Anyone that is paying that much for a extreme GPU should do research a realise it was due a short lifespan at the top of the tree with the 16nm imminent and will lose value.
But unless you intend to stop PC gaming you haven't lost out. If you feel (after we see true benchmarks) the 1080 is a better option, sell the Titan, buy the 1080. There will be Titan buyers for 4k SLI, so it's resale price should be around the same as a new 1080. (There aren't that my Titans to begin with so the price isn't going to drop massively.)
The 980Ti will lose more value, but again it's relative. Sell the Ti spend £50 on a 1070 if you think it's better (Sell now and you might get a 1070 for the same price or less), or £200 on a 1080 if we do get a 30% performance increase.
If you don't think the upgrade cost are worth it, don't sell, you no worse off.
Here's a rough snap shot of how long NVidia top card have lasted before they have been beaten by other NVidia Cards.
GeForce GTX 480, March 26, 2010
GeForce GTX 580, November 9, 2010 (228 days)
GeForce GTX 680, March 22, 2012 (500 days)
GeForce GTX Titan, February 21, 2013 (Not released as a gaming card) (337 days)
GeForce GTX 780, May 23, 2013 (427 days from 680, not Titan)
GeForce GTX 780 Ti, November 7, 2013 (168 days)
GeForce GTX 980, September 18, 2014 (315 days)
GeForce GTX Titan X, March 17, 2015 (This time a gaming focused card) (180 days)
GeForce GTX 980 Ti, June 1, 2015 (272 days from 980, 92 days Titan X)
GeForce GTX 1080, May 27, 2016 (361 from 980Ti days)
GeForce GTX 1070, June 10, 2016 (375 days from 980Ti)
Looking at that, that a reasonable life span before being superseded. And in most cases the 2nd top card (670 etc.) were around the performance of the previous top tier card.