Higher value cards depreciate more then lower value cards. They don't retain their price better. That's the way halo products work as they aren't a good deal when you buy them and they're even less of a good deal when you're buying them used as there are new products on the market which supersede them for cheaper. Sweet spot is always midrange for new and used from a consumer standpoint.

As far as mining goes, 1080/tis don't scale properly in a lot of algorithms and are crippled in memory hard algos such as Dagger from GDDR5 X. They have heat dissipation issues due to the size of the chips and will throttle a lot faster then a 1060/70 on algos they do work properly on. The size of the heatsinks required to cool them properly are in the three slot variety. They also consume more power which means you need to have a beefier power distribution system for each computer, which gets even more expensive the bigger the PSU.

Did I mention they can't really mine dagger? That's a pretty big deal. It wouldn't surprise me if we're back dual mining Eth again on Nvidia hardware. That already happened earlier this year. Also why AMD hardware is just outright better as far as investment goes for mining (if we could get it remotely around MSRP). For comparison a 580 is making $2.51 dual mining and a 1070 is making $2.35 mining Zcash. Actually looking at prices right now, you can get 580s for $350... So it still is a better deal. As hardware values come down from their bloated levels it'll become a even better deal.

It blows my mind people still can't compare Nvidia and AMD properly. Guess they just see the price tag for a 'mid-low end' card and they can't put it into context when it comes to mining, just playing games.

Other things in the crypto scene being more beneficial to do is outside the scope of buying hardware. We can discuss how much money Claymore is making and the fact he doesn't have competition or that we still don't have the equivalent of Claymore for Nvidia.