My bet is that these >50K sign up bonuses won't last very long. We've already seen several cards being reduced (AA 75k->50k, BA 100k->50k), and a crackdown on churning by Chase and Citi. These >50k signup bonuses are actually a rather new (2010) thing and what they've done is inflate the currency. Something has to give: either a devaluation of the miles or reduced signup bonuses.