I agree this has to be some ASIC in play, because there are no new 200k GPUs in the world.
Given how hard GTX 1070 cards have been getting hit in the last week, I disagree with that last statement.
I don't see the "difficulty bomb" mattering, and don't understand why ETH wasted the time to create it - as it affects all miners equally, it isn't going to change profitability at ALL on a per-card basis.
The "phase in" aspect of the current plans to move ETH to PoS might soften the landing when all those ETH cards start looking for new homes - but it's still going to be brutal eventually, unless the current price jumps resume before that point in some of the other bigger coins like ZEC.
We're also talking sometime NEXT year it looks like before it becomes a significant issue - though ETH has had to "delay" plans for the PoS transition before....
It's not stupid for a miner to sell their coins immediately - it's sometimes "I have BILLS to pay", it's more often "I don't trust this coin to stay at this price" or "I don't trust the volitality".
Keep in mind that coin prices are NOT automatically going to go up - Bitcoin was well over $1000 at it's earlier peak, then dropped as low as $200 and spent a LONG time under $500.
Litecoin hit it's all-time peak near $50, a year later it was more like $2 and spent a LONG time under $5 - and I don't think it's EVER gotten back to that all-time peak (though this last week it got very close, one way or the other).
Then there's the bear in the woods that far too many cryptocoin devs keep ignoring - outside of Bitcoin, ETH, and Litecoin, how many coins have significant acceptance in The Real World?
In fact, I probably shouldn't include ETH and LTC in that list as both combined don't get accepted at 10% as many places as Bitcoin alone does.
Without the ability to spend the coins in The Real World, they have zero chance at long-term viability.