The ACR 406 is a great PLB. It does one thing and does it very well and is relatively inexpensive. Satellite communicators (Garmin and Spot are the industry leaders) have become more and more common because they are more versatile: most allow for two-way text communication and tracking features allow those at home to see your progress and location, and you still have an SOS button. That is where the industry advertising $ seem to be focused. Just depends on what you want out of your device.
$80 is a good price for a battery replacement. I paid approx. $125 several years ago. The replacement had a 6-yr life, better than the 5-yr original battery which was already a year old when I bought the thing. Personally, I would not gamble with using the device past the battery life. Yeah, it could work, but seems like that kind of defeats the purpose of a piece of gear that your life is depending on.