One more issue that rarely gets mentioned, is the capacity of the EOGs and the HOG. Adding more coaches adds more electrical load on these systems. They may develop unwanted harmonics (secondary characteristics in the power supply) because of the increase in the rake length.
This would be an even larger problem to handle as all the EOGs and the HOG systems in locomotives would have to be upgraded to the new specifications.
This is potentially an issue in this case as, particularly in IR, we have a tendency to always...
more... implement systems that 'just' meet' the needs. We usually leave very small margins for updates and expansions.
Most stations have platforms and loop lines that are 'just' long enough for the longest rakes of that time. The locomotives are typically just powerful enough to handle the largest loads of that time.
It is such philosophy that is the main hurdle in transitioning smoothly to newer technologies, and optimizing operations (in this case, increasing the LHB rake length).