Well the parts used in the post cost $23.12 so the IoT button is cheaper.
And unless it's changed, the battery was soldered directly on so it's basically not replaceable unless you really know what you're doing.
The new ones have a clip on the battery. But the case is still closed with ultrasonic plastic welds.
IIRC you have to sniff for its ARP probe, it works most of the time but it's kinda finicky. An ESP8266 is a similar price.
Yeah, i don't think the Dash Buttons were ever hackable other than by intercepting it's network traffic. And there isn't really any way they can stop that.