That graph has a couple doglegs that make it look exponential.

The last dogleg was January 2015. And since then it’s been linear (with a little stall this month) at about 5% of the Internet converting per year. That’s another 15 years to convert the rest, unless there’s a new dog leg up.

Also percentages don’t work the way humans think they do. Especially when the number of devices is constantly climbing. That may just indicate that some fraction of new hardware is ipv6 but little old hardware is being updated.

We may well have ~2 billion machines on ipv4 pretty much indefinitely, slowly being diluted by addition of new hardware.