From the Quora question:
The attack sould last 1h, spending those 400 BTC for 80 times instead of just 1
You can't spend 400 BTC 80 times in 1 hour. If you control a majority of the generation you could spend them twice an an hour (assuming merchants require 6 confirmations).
So you need to divide your expected profit per hour by 40, making your ROI very, very negative.