I think a part of this is that engineers in particular have a preference to use software which can be treated as a transferable skill if/when they move on. They would rather use the OSS version of Nginx, of Envoy, because they know they will have access to it in the future. I think there is some aversion to becoming familiar with the features, functionality, and characteristics of a piece of software that your current employer is paying a non-trivial amount for, when you know that chances are your next employer will refuse. This may not be in the best interest of the current company, but it's a bias that I think impacts a lot of engineers.
You can purchase the commercial version of Nginx and not use its specific paid feature subset. Alternatively, "I am familiar with this, and we can do x% of what we need with the OSS version, but we had to pay to get the last part."
I see what you're saying, but this is basically suggesting that Dropbox should have made a donation to F5 (a public company with an $8B market cap).
I think there is a valid point you're making for smaller companies that are providing both open-source and commercial versions of software, but I don't think Nginx is a great example of that.
