Actually the market for new sub-$20k vehicles is quite small. Most people with budget concerns buy used. The median new passenger vehicle is something like a $40k minivan or pickup. So these EVs really aren't very far off the mark financially.
I seems to me probably another 5 years to get enough volume in solid lower end EVs that are bought in quantity and have a decent lifespan. Then another 5 years for them to filter down to the used market with decent prices.
I bought a 2017 Golf last year for around $15k. In Australia the only thing you can get electric wise at that price point would be a 2013 or so leaf with 100km range, and even then there is basically no supply.
You can get a ~2016 Leaf for around $10k though.
It's ideal really; those who can buy new eat the depreciation, while those who can't afford new end up with a vehicle that will be able to go hundreds of thousands of miles before end of service life (perhaps with some cosmetic blemishes, but not much more of concern considering powertrain longevity). Not everyone needs a new car, many simply need a reliable car.
Consider: I had my last car purchase (which I knew would be used) between a BMW 4 Series, a VW Golf R, and a Subaru STI. I went with the 4 Series. The Golf R and STI are often used as track cars, and 40k miles on a Golf R may very well be the equivalent of 100k miles on a regular Golf.
I admit this is a very specific use case, but there are other specific use cases (e.g. police car - probably spent 80% of its engine's life idling and not racking up miles) that we know to look for, that we simply don't know to look for in the EV market, because they haven't presented themselves yet.
But yes: batteries age and need to be replaced. But while that's quite expensive in materials, it's actually very feasible as a maintenance task. A 2012 Model S could have its battery swapped and no doubt drive for another decade with minimal fuss.
So a bunch of EV's sitting 20-30% above that level aren't being priced out of the range of popular cars. They're very feasible purchases for most buyers.