The title is a bit misleading. 2.5% coupons are nothing new--especially if we're talking about 15yr MBS. Even in 30yr MBS, 2.5s were being traded as far back as 2012. This is only logical considering ...
Let's do the simply-worded conclusion right up front. For fans of low rates, the Fed's inclusion of 1.5 MBS (30yr) is a good thing, but not some new and wonderful guarantee of ongoing improvement.