That's not necessarily true. People don't die when twitter is down, and whatever twitter's business model actually is, I am not even sure there is a monetary penalty to them being down (unlike, say, Amazon being down which results in lost orders). They may have made the calculation that it was not cost effective engineering-wise to chase that extra 0.001% of reliability.

[Edit: Pedantry shield: Ok, ok, should have said people don't die because twitter is down. Obviously people are dying all the time, and some will indeed expire while twitter is down].