I don't think I would pay for any of the core features, but that's mainly because I've been using Docker for 5 years with the current strategy of just about everything is open source. You pay if you want support and other enterprise features. Private hub repos are also reasonable to buy for most people because it's priced pretty competitively with rolling your own private registry on a cloud provider (ie. $7 / month for 5 repos isn't much more than $5 / month for the cheapest DO droplet but now you need to manage that machine yourself which is not worth the hassle for $2 less).

I would pay for an ecosystem of images that are vetted for security issues. They could even build an industry around security and privacy compliance and certification, which risk-averse companies (and those who want to do business with them) would pay for.

To be fair, the article is about both. To quote "in my humble opinion the days for Docker as a company are numbered and maybe also a technology as well."