Interesting that Hannigan hasn't said anything yet.
She's currently contracted to Disney with a new TV show soon to air so I would guess she is under all sorts of social media restrictions right now, especially in the wake of the Gina Carano stuff.
Charisma Carpenter was purposely killed off as soon as possible after she had delivered her baby (can't fire a pregnant women I guess) and found out that she was being killed off from a journalist rather than direct from JW. As well as the humiliating storylines and total sidelining (her character was off-screen in a coma for large parts of the season) he also employed tactics such as calling her in at the last minute for 1am reshoots despite a doctor saying she needed to rest and berating her on set for gaining weight.
There are all sorts of stories and anecdotes about the various productions that show he was hiding in plain sight all this time.
The cynical part of me knows it isn't going to damage his career. At some point soon he will release a repentant statement that will borrow heavily from the Script Of Caught-Out Abusers. The statement will reference his "passion" for his work and that sometimes this passion will spill out into frustration and frayed tempers, he will say he is sorry if his actions offended anyone (making it their fault for being over-sensitive rather than his fault for being being abusive arsehole). He will say he is learning/growing and always strives to "do better" so will take lessons from this situation (i.e., don't hire anyone in future who is likely to blab). He'll go for show-therapy or even for show-rehab to get help for his "anger issues" and then his career will carry on as normal. He'll be a "reformed character", welcomed back into the fold and lauded for "facing his demons" with no end of people willing to work with him (for reference see Woody Allen, Roman Polanski, Bryan Singer, Michael Douglas, James Franco, etc). Meanwhile Ray Fisher, Charisma Carpenter, and all the many others will have lesser careers because they'll be known as "troublemakers" and "difficult to work with".