The way I see is that as etc gone up, it will not be going anymore up till other coins go up too, so my theory is if you mined or have etc then sell or stop mining it, mine or buy other coins, you will have better chances to profit a little bit more.
This is exactly what happened in the past 24 hours. My Tesla P100 rig on Minerstat auto-switched from ETC to ERG last night. ERG profit is currently $1.73/day while ETC is $1.51/day, mainly because ETC uses more power.
But the effect you're describing can work in reverse. Once ETH goes to zero after PoS, profit on other coins will crash too. That is when 90% of miners will go bankrupt and panic sell video cards.