Right now the margins on gasoline vehicles are much better than electric vehicles, partially due to the technology being relatively immature. The internal combustion engine has more than 100 years and hundreds of billions of dollars in R&D behind its infrastructure.
Apart from that, most dealerships (which is not the same as the automobile manufacturer, but they are closely related) make most of their money from maintenance and servicing vehicles. One of the big selling points of electric vehicles is that they're simpler and cost less money to maintain.
A cursory analysis suggests your opening statement is actually wrong - there will be less revenue if there is widespread shift towards electric vehicles.