You don't need to retrofit, you can just build a reasonably-featured car to begin with. I imagine most people just want a corolla with a battery (or cheaper).
Trouble with my ('95) Corolla is that it's the body that's showing its age. (Rust, and not the good kind.) The ICE is just fine and probably good for another 1/2-million km, but the whole thing has got to go soon.
"Let it rust"? If you live near the sea, cars rust and there's not a lot you can do about it. You can attend to visible rust timeously, but there are plenty of hollow spaces where it can fester for years, all unnoticable, before it pops out through the paintwork.