Again, there are no batteries to be replaced every 8 years, that's just when the warranty expires. Do you expect to replace your car's engine when it's warranty expires?
Virtually no batteries have been replaced on priuses, except for manufacturer defects. A cabbie in Canada drove over 200,000, and Toyota took his battery and gave him a new one to study, and found that the battery was "like-new".
The battery is not a critical part of the system (in regards to performing in like-new fashion), they are just a temporary storage medium for energy that would otherwise go to waste. The batteries are operated in a narrow range, 40-80% of charge, and in those ranges the batteries suffer no noticable depreciation in capability. But if they did, and after 10 years you were using 30-90% of the range, you would still not have to replace the battery.
Meanwhile, the system is extremely simple from a mechanical point of view, it doesn't even have a transmission, the brakes get less wear, the gas engine is never fired unless it is already spinning at 1500 RPM and the oil is pumping. The only complication is the computer software.
I've driven my 2002 Prius over 50,000 miles, and the only thing we've replaced are the tires. Realise that the engine even runs much less than normal for the mileage so if normally the engine would last 200,000 miles, the Prius engine should last 300,000. Electric motors virtually never wear out -- these have no brushes, and the bearings are replacable but don't get a lot of wear either.
If you started with an electric car, and decided to add a gas engine for recharging, and then figured out what the best way was to use it, you'd end up with the Prius.
2006 Prius. 1021 miles on the odometer since we bought it new. Traveled to PA. Started with a full tank, filled up after driving 136 miles, using cruise control where feasible and with the air on. Took 2.2 gallons. More interior space than my '99 Camry. Wife tells me to slow down when I start cruising at 80. I see drivers of SUV's and similar trucks/cars trying to keep their speed down and even to squeeze a few miles more out of tankful so they can postpone the $50 to $60 fill up a few days longer and I laugh.