Granted they're dumping the fleet with degraded battery packs before doing the 15k+ replacement, sure.
Most American company ICE cars are scrapped by then. Not to mention all the ICE related maintenance costs over that time like oil changes, transmission fluid, timing belts, hoses, etc. etc.
You can look at 6 to 10 year Model S cars.
Buying an EV with 100k miles and the original battery pack for 20k is a bad idea.
The Prius battery is far smaller (about 5-10x smaller) than a Tesla battery, so it is cycling charge / discharge far more often.
So you'd expect the same quality battery to last 5-10x less miles in a Prius than a Tesla driving the same distance. It's likely that Tesla's investment in battery quality and active temperature controls mean the Tesla battery is also treated better and higher quality.
Topping that battery up with a generator in the Prius complicates things, both reducing kWh used per mile (some from generator) but also allowing a discharge - recharge cycle to occur multiple times in the same trip.
I don't think you do. Prius's have only recently started using lithium ion batteries, for one. And they cycle the small battery a lot faster than a BEV. Everything we've learned from hybrids suggests BEVs will get hundreds of thousands of miles from the battery (and so far, from the cars which haven driven that far, this has proven correct).
I have a Tesla, and I use it in the worst possible way for the battery: road trips with deep discharges and frequent 100% fast charges. I'm at 100k miles, and it degraded by 9%.
Politics.
Right now if I had to pay that out of pocket it would be more than a new engine, but not by a huge amount. By the time I don't have a battery warranty, the cost will most likely be in line with a used engine for a similar ICE car.