Nope.
The HCH-2 will not run forever if the traction battery fails. While it may run for a while, that condition is not sustainable at all. One of the reasons why is because the 12V needs to be recharged since without it the car wont even start.
With regards to the price of a new battery pack:
The retail price for a battery pack does exceed a little over $4000, and that does not include a new BCM module which BTW is often part of a battery replacement.
Now for the sake of being realistic: In almost all of the battery replacements performed on the HCH-1's that exceeded their warranty coverage period, Honda has been stepping in and paying for half of the costs for the battery replacement - often HCH-1 owners are expected to pay anywhere from $2300 to $2500.
And since this will remain the standard approach for the foreseeable future there's no reason to fear a $4000+ cost for the battery replacement at all. Now if you buy a salvage title HCH-II or some other unit that has lost the manufacturer warranty then yes, expect to pay the full fare for a pack replacement.
Lastly, we are often talking about the better known battery replacements on the HCH-1. That does not infer or relate into the life expectancy of the HCH-2 battery pack at all. The HCH-2 battery management routines implement much of "new knowledge" that Honda did not have when they designed their Insight and HCH-1. Also the manufacturing and packaging of the cells has improved as well on the HCH-2.
So there is no reason to believe the HCH-II battery will be not be reliable and dependable for most owners

.
By the way, Toyota learned the same way on their Prius 1 and factored their new found knowledge into their Prius II platform.
Cheers;
MSantos