I do agree with you on this matter - I do not wish for 100% electrification. In addition to the points you mentioned - A single WAP7 consumes about 4.5MW of electrical power. The energy consumed by the locomotive in one hour, is sufficient to power about 50 average households for a month. With increasing electrification, power companies will be strained to supply the large amounts of power required. Especially during the summers, this comes at the cost of power cuts in villages and farms. So, indiscriminate electrification is definitely bad.
That said, as far as the construction of a new Pamban bridge is concerned, the primary reason of the construction is not electrification. The bridge was constructed for meter gauge traffic...
more... more than a century ago. During gauge conversion, they considered constructing a new bridge for BG, as the old bridge was constructed for MG and was already about 90 years old. But, due to insufficient funds, they had to just rely on strengthening the old bridge.
In recent times, several engineering works had to be carried out on the bridge as faults were observed. So, they decided that it's high time they constructed a new bridge. Electrification is just an added bonus.