I've had a hybrid car since 2006. The battery system is indeed expensive to replace. Eight years was about how long my car went before the battery system (it is comprised of 100 lithium ion battery cells) was no longer holding a charge for very long and was relying almost solely on the gasoline engine for acceleration. The manufacturer (Honda) replaced the battery cells without charge though since they had determined that the software that controlled the battery system was not configured properly and caused the batteries to be maintained in a sub-optimal way, which cut the battery life short. Because the batteries were under recall due to this, they replaced them all at no charge. So while I got lucky there, it does also suggest that the batteries (in this hybrid at least) are intended to last longer than 8 years. Perhaps with a pure electric cars, they are not projected to last as long because there is so much more charging and discharging of the battery.
I am not sure about this, but I have heard that Tesla is recommending replacement of the battery for Model S, every eight years. But then, Tesla is also subsidizing the replacement.
It seems to me that hybrid cars is a dead end. With the price of batteries even for use in the city, these cars will be too expensive. I think that the situation will not change until not come up with a new type of engine which will be cheap to produce electricity.