There's no reason why it should be short term. Federer has had a mixture of huge form dips and minor form dips. However, each dip has been accentuated by stronger competition over the past 4 years. His 2008-2010 form would still have carried him to 3 out of 4 slams against the 2004 field, despite a noticeable loss of form.
I don't see any physical decline whatsoever and see no reason why he can't sustain his current form.
Refer to my previous post. I pointed out signs of his decline, and also reasons why his current form is a bit deceptive actually. Until he wins another slam, he's just continuing his steady, slamless decline.