Responsible economics should be based on replacement costs. If Caltrain dumped the whole fleet just to get EMUs it would be a dumb bureaucrat move - SURPRISE!
The basic econ could work if EMUs were used to replace cars that were truly up for replacement. and locomotives to haul cars that had significant service life remaining.
[aside- the newspapers around 1910 reported that all the SP Bay Area commute lines were to be electrified, but only the O-A-B lines got electricity]
All that said, look at the east coast.
Ca 1915 PRR replaced locomotive hauled trains with EMUs to reduce the number of moves at Broad St Station- no cutting off engines, turning them, and coupling to the other end. When the Metroliners were invented they were EMUs- to be faster than the trains hauled by GG-1s, a little. But then replaced by locomotive hauled trains. Say aren't a few of those original Metroliners de-motored and serving as cab cars on Amtrak?
But then that last decision was by bureaucrats as well.
Perhaps someone can share the story, including gory details, of the change from EMU to AEM-7s etc.
Besides, the Caltrain electrification scheme is partly based on money stolen from HSR, that CA got from the Feds when Florida said "no thanks-we can't afford it" P--s it away or lose it.