the thing is the batteries and electronics do last. And thus the cars in the yard are collision damage. People don't ever (practically speaking) need to rebuild/replace an inverter, and the batteries don't fail hard, they just lose some capacity with time. So you have an abundance of used parts, and really the only market is the diy market, which isn't huge, so parts wise the costs should keep getting better for the DIY'er as supply is much greater than demand.
But as resale vs battery life, that is mostly political. There isn't really anything to it. Deciding to renew vs repair vs just drive it is a simple matter of time/cost analysis vs mpg recovery. It still runs fine with a low capacity battery. And again, getting to the battery (on a prius anyway) is fairly easy
Here is one with an 18 month warranty for $850!
2004 2009 Toyota Prius Hybrid Battery Complete w HV Computer 18 Month Warrnaty | eBay
Admittedly not everyone is up for replacing a battery let alone identifying and replacing a bad cell ($30 repair), but it isn't even a dirty job. And lots of good parts, and not seeing a lot of reports of hard failures. The apprehension is largely because of the "unknowns", but look into it (beyond resale apprehension) and learn something about electronics, and you will see it is the least troubling aspect of it all.
The other thing to consider is battery technology is constantly advancing, and really it is just volts and amps and capacity. It isn't very far fetched at all to imagine upgrading your old hybrid battery in the future cost effectively.
edit: that $850 battery comes with FREE installation, so really, no excuse for excessively low resale based on battery speculation.