Yep, cars depreciate rapidly in the first few thousand miles. Less if it's from a company with a good track record and not just good branding. Nothing no one knew before.
People get prematurely excited before they really figure out what the trade-offs to new technologies are. Engineering is hard, mmmkay?
"As to complete replacement of existing cars, it will have to be something so major the we are willing to throw away all of the existing, functional, existing machinery. "
Only if you live in a vacuum.
Consider this probable scenario..
If existing electric cars have the same exact performance 100 years from now as they do today, but advances in their production and increasing gas prices make them cheaper than IC alternatives... what do you think will happen?
Here's my guess. IC cars will drastically increase in price due to lower production volume. Remember when automatic transmissions cost extra? Lots of supercars now-a-days don't even offer a manual transmission. The 'standard' flappy paddle alternative is less reliable, more expensive, harder to maintain, less efficient, etc. etc. yet it became the 'standard' because of customer demand. IC cars will be the wind-up fine Swiss watches of the future. Luxury items.
OH NOES! EPA! Been watching some Yellow Papers I see.
If I were you I would remove all airbags from my car and apply aluminum cranial shield ASAP.