Just as a point of order, SM has been partially retracked on a couple of occasions.
As for B&Ms, expected life span would be about 20-25 years without the need to retrack under ordinary circumstances. I don't know or pretend to know the reason for some sections being removed at the moment, but other B&Ms with onboard audio don't have a different gauge of track to support the extra weight so it won't be replaced for that reason. Bizarro at SFGAdv and Hollywood Dream at USJ both have onboard audio with exactly the same track structure as other models of the floorless and hyper respectively.