This is the nugget of the issue for me. However you define “too much,” dead weight is precisely what a diesel engine, its cooling system and fuel tank is, when the train is running under wires. More energy is required to stop and start, and when you add that up over a lifetime of journeys, it’ll be a lot of wasted fuel and more cost passed on to passengers.

Having said that, I think the major fall-out over this is likely to be continuous delays in actually finishing the electrification projects that have been promised. Purchasing new bi-mode rolling stock because the electrification project is a few years behind schedule is likely to give them decades (the lifetime of the stock) to ignore the issue, because “the trains can run on diesel power to cover the gaps”.