I also doubt Hive would suffer another exchange attack, but, if it did, I suppose it would require a hard fork to mitigate within the 30 day window for witness voting

The exchange attack wasn't enough, it was combined with the fact Justin Sun got 76M under the table without affecting the market. This will never happen again as no one party owns even close to that. I doubt exchanges will ever fall for that again I'm honestly surprised there hasn't been a legal action taken against them for their involvement and what they did to customers funds. Never too late I suppose.