'Everyone knows' that all altcoins will crash during the Bitcoin halving. ಠ_ಠ
I think that only goes for small and short term changes in price likely because when people see that Bitcoin's value increases they tend to dump alts in favor of BTC and cash out in FIAT to maximize alt to FIAT value.
But if Bitcoin were to go mad and get to something to like $700 or so then it would definitely bring the altcoin market with it because at that BTC price a whole lot of new people would get interested which means a whole lot of people would also get interested in alts as well.
Bitcoin will die after halving, price wont increase much and it wont be profitable for most people to mine so the mining will concentrate in china. I mean, the halving hype is already included in the curent price since it wasnt a secret for anyone.
I think you underestimate the major miners by a large margin. They know, they are prepared and while they will probably wait for a rally to sell (even if that's maybe more than a year from now).
And likely they would still be profitable if they were sell every day after the halving. I could be wrong though but I think the big players got it down.