Ive got internal routing on my BmC for both my shifters and brakes. Never really been an issue for me, for the once a year I replace the cables. I just slide a liner over the old cable, then slide the new cable through said liner. Very easy. Not sure how you would do it otherwise, with magnets and stuff?

I have noticed the cables inside the frame are susceptible to corrosion. So the argument that internal cables last longer because they arent exposed is not true.