The concept behind Twitter? No. Twitter? Maybe..? They have a user experience they want to deliver to 9X% of their users, so they optimize for that. For most of Twitter's users, their connection speeds aren't the limiting factor in their experience. If you optimize for the lowest common denominator, that 9X% almost certainly gets a worse experience.
* There is no proof in your statement that currrently 9X% of their users have fast internet connection.
* Even so, it would simply be an artefact of them never bothering to optimize for people with slower internet connections. If they did optimize, a lot of their traffic would come from the so called "lowest common denominator" just as the blog post says it did for Google.
* Finally I find the term "lowest common denominator" misrepresentative because it implies that optimization needs to cater to the slowest connection on earth, which is clearly not the case. If the average speed is above 90% of internet connections (as per Akamai report cited in the blog post) then the distribution of internet speeds is clearly skewed and there's value trying to even cater for median if not the minimum speed.
It's not always a tradeoff. Several sites stopped serving CSS during the Superbowl (https://twitter.com/jensimmons/status/828415747625992192). At a smaller scale, the same thing happens to some sites that reach the HN front-page.
It depends on exactly the costs are. But in that example, those sites are spending millions on Superbowl ads, and it's probably their highest day of traffic, so it's not "almost no benefit".
