People aren't doing this for fuel savings. It's a hobby, or a technical exercise. It's just a more modern version of dropping a modern V8 into an old flathead, something akin to old-fashioned hot rodding.
The folks that are dropping 250,00 (and sometimes quite a bit more) into various resto mods and drivetrain swaps (some of which are ICE and others EV) aren't doing it to save a few dollars per year on fuel.
Tell me, when you see a deuce coupe or roadster with small block Chevy or 302 Ford backed with a 4- or 5-speed box, do you think of the horrific transgression that took place, the blasphemy of destroying an old classic or do you appreciate the creativity and workmanship that went into transforming a clapped-out rust bucket into an enjoyable street rod?
I appreciate a fully restored old car, or a survivor, and have owned one of each, but I also dropped a '58 Olds Rocket 88 mated to a Hydramatic into a 1954 Ford F100. They were all useful and fun-to-own-and-drive vehicles.