I’m not sure I follow your logic. The environmental impacts of manufacturing will be averaged out faster with something that’s getting a lot of use, and targeting the efficiency of something that gets continuous use has more impact.

Throwing away busses to switch would be silly, but it’s a perfectly good idea to switch as you maintain your fleet. Your alternative is buying more diesel busses, which… No, building a trolley or tram system which is more expensive and less flexible, or rebuilding cities to be livable without needing vehicle transportation, which isn’t happening soon and has trouble for people with mobility issues like the elderly without other some accomodation.