This sort of thing assumes that we are all on mobile devices with wifi.

I'm on a desktop, with a hard wired ethernet connection. It's also my work machine. To go 'offline' I'd have to either pull out my network cable, or go into network connections and disable the NIC. My work email would start complaining, and before long someone from IT will probably show up to see why my machine has fallen off the network.

As others have said... there's also Offline Mode in Chrome.

Now, imagine I'm a less techie user, using the browser that came with my work PC. I'd have no idea how to go 'offline', or even worse, be physically unable to. As such this content is out of reach to me.

I hope this sort of trick doesn't become a trend. There's a big difference between 'not connected to the internet' and 'offline'.