The other problem is that it's not so easy to upgrade now because hardware is so overpriced. 1+ BTC per TH/s - nearly the same as it was a year ago, I paid ~2.5 BTC for an S4 back then. The S7 (or the Avalon) has ~1/3 of the power consumption of the S4 but even with that in mind the end result still is: I'm going to mine less BTC per each BTC spent on hardware that I did a year ago, which means that hardware would need to last longer to ROI, which seems unlikely with faster rising diff and halving.
This is part of why I like keeping some of my holdings in gear. It by far is not a sure thing, but I enjoy it. And it's been good to me over the years.
But S4's looking back were a really good deal. I mined with mine for a LONG time through summer even, they were part of reason I got a mining area. But after all that mining they hold a amazing amount of their value still. So those who had S4's (if decent electricity) have done well I think. And selling it eventually is the cherry on-top.
Yep. Also SP20 at $500 last December. I sold the last few just recently... for $500 each
Somehow I doubt I'll get $1500 or 4 BTC or anything close to that for the S7 or the Avalon6 a year from now.