Absolutely correct. Given that they're in California and the house wasn't new construction, we can pretty safely assume that it was under-insulated compared to

Absolutely correct. Given that they're in California and the house wasn't new construction, we can pretty safely assume that it was under-insulated compared to what's possible. Doing a proper audit and fixing air leaks and insulation is usually the most cost-effective improvement you can make to a house (getting it right to begin with is even more cost-effective, but lots of builders still don't know what they're doing). As sibling notes, it can be invasive and disruptive vs. just replacing some appliances in a few days, but it's still more cost effective (and will allow your eventual replacement HVAC system to be sized smaller, netting additional savings when you do that).