Presently about 35% route kms (& even higher % of track Kms) of IRly network is electrified. Once the electrification level reach about 60-70% of total tracks (based on the traffic), the left over non-electrified stretches shall be of short distances. These small non-electrified stretches shall become operational bottleneck due to frequent loco changes required and subsequent delays and reduction of line capacity. At that point of time electrification of these leftover stretches shall become a necessity. Ultimately IR shall have to go for almost 100% electrification for BG tracks.
Any how it would be unwise to scrap diesel locos altogether. Some has to be maintained to take care of emergencies due to breakdown in OHE lines.