That wasn't their reasoning at all, they allowed a subset of accounts to have 280 characters for several weeks. During that time they did quantitive analysis that showed only 5% of tweets sent by those accounts went over 140 characters and 1% went over 190 characters. That's why they think most people won't use 280 characters. However increasing the number of characters allowed alleviates a problem where 9% of all tweets in English ran up against the 140 character limit.