and perhaps I should also mention that when I sold my 2nd Gen 1 Prius at 9 years old with 163,000 on the clock, it still had an original 12V battery. Also, the engine and HV battery were performing perfectly, except it had started to need a little oil after 100,000 miles (by the time I sold it, it got close to MIN about 7,000 miles after a service, and 1L carried it through to the next.
The guy who bought it traded in used Hybrids and also maintained the HV systems. It became his wife' car for a couple of years and he then sold it to a customer. I got the odd update when he serviced it once a year and apparently at 180,000 miles and about 12 years old the HV battery started giving problems. He dismantled it, checked which sticks of cells had failed, matched them with spares from written off cars he'd bought and AFAIK the car is still in daily use.
He has reported a few other cases of HV battery failure on 2000-2003 Gen 1s and a few 2004-2006 pre-facelift Gen 2s, many of which he's been able to rescue.
My own dealer had a customer bring in a Gen 1 with a failed HV battery and he didn't want the cost of fixing it and asked the dealer to dispose of the car. I put them in touch with the guy who bought my Gen 1, and he got the car to drive well enough to get it home and fixed it up.
I'm also aware of two cars that had the HV battery killed after the driver put diesel in it, and let the Hybrid system try to start the ICE so many times it flattened the battery enough to terminally damage it. Once of those was a 2006 Gen 2 and in 2007 the cost of a new battery had fallen to about £2,200 (from around £5,000 in the very early days).