May also be a problem with ducts from the exchange to the 'Cab.
If say they decide to add another 100pair E-side cable that will depend on how full the ducts are since a 100pair will be a decent sized cable.
There are various calculator around that can work out the % fill capacity of a duct depending on x,y,z etc
If it simply is not going to fit then BTOR have a major problem.
If the logistics and the number of subscribers allow...
Might be able to pull in a smaller new temporary cable, swap all subscribers on the mostly dud cables to that new cable on a temp basis then pull out the duds and then install a new 100 pair then swap all to the new cable and them remove again the temp cable.
Probably less work (marginally) than installing a new 4km length of ducts!
If they can get away with installing a 25 pair and they can then they will and any future problems down the road with insufficient spares or yet another cable already in the duct when the others pack up then that will be someone else's problem in 10 years+ time.