Realistically, if it really can deliver a $12K per year cost savings, and if it can last 20 years, by the books that's a $240K savings over 20 years. From an accounting perspective, that's pretty attractive considering how problematic some of the modern diesels are with the emissions and such. The question is, can they really deliver on the cost savings over 20 years?
A similar scenario played out roughly 40 years ago, when buses were primarily gas powered and diesels were considerably more expensive. The diesels had a tendency to last longer, use less fuel, and generally needed less maintenance and proved to be cost savers. Will this happen with the electrics?
Of course, even the Ford Edsel sounded good in theory and on paper. We all know how that turned out ...