I don't think they're close to adoption. There are 7Bn phones in the world and growing. Granted a chunk are corporate which won't ever need WhatsApp, but most in third world countries still use burners.

And although there is likely a slow take up by the elderly, the next generation are all new customers. The 1/5th smartphone ratio can only go up. (This POV is of course assuming WhatsApp wins the messaging war, which is what FB is betting on)