They don't need to sell to altnets. There are plenty of big players like Amazon and Microsoft who are hoovering up IP addresses for use in their cloud services.
Of course, the longer Zen hold out, the higher the price will go. But they have to cash in at some point.
We seem to be a long way from the tipping point where IPv4 addresses are no longer in demand. This will only happen when:
1. The vast majority of the content on the Internet is reachable on IPv6; and
2. The vast majority of users on the Internet have IPv6 access
(1) is held up by huge corporate laggards like the BBC (who used to be technical leaders back in the 20th century). (2) is held up by ISP laggards like Talktalk and Plusnet.
At one point there was talk of China switching off IPv4. If they do, that may force the content providers to start enabling IPv6 access, or lose about 1/5th of the planet.