Not saying its likely but imagine if Federer lost early in a couple of slams, that would lose massive points since he's always defending big points, so I wouldn't rule out the possibility of him slipping not just from number one but from the top2 in a hurry, plus he's unlikely to make it up with Masters wins when he does lose the slam points
it may not happen this year (or it might) but it will happen sometime and when it does nobody will be expecting it
actually I imagined that situation because if Murray wins Wimbledon (which is not impossible at all
), Federer will lose many points there.
And if Del Potro did better than Federer in the beginning of the year, and even with a small result in Wimbledon, he would be ahead of him.
Yes Federer depends a lot on his points in slams (and more especially in Wimbledon which is his favorite one), that's for sure