So if you're in a country where it's economically feasible to continue maintaining cars beyond 15 years, why wouldn't you think that the much cheaper electric cars wouldn't dominate the market even more?

Compared to the U.S.'s low adoption rates, EVs sell at higher rates in certain rich countries like Norway and Denmark, middle income countries like China and Turkiye, and poor countries like Estonia and Nepal. The cheapest EVs, globally, are cheaper than the cheapest ICE vehicles.

And that's a problem for ppl who buy 20 year old cars.

The cost of maintaining a car to last 20 years can be applied to EVs and ICE cars alike. I suspect that EVs will be easier to maintain to those ages. In the U.S., that doesn't really happen in large part because our labor and parts network is expensive enough that buying new is comparably cheaper than repairing, past the 15 year mark, for most vehicles. EVs don't actually change the equation any.