Not developer program revenue. At every turn where they have a choice, Apple does the same thing: sell devices increasingly locked to their own ecosystem, and sell it as a feature to protect folks. This strategy works really well: they actually have folks arguing that giving all Apple users fewer choices is a feature.
They have been ratcheting down on freedom in OS X (and then MacOS) since 2011 or so when I stopped using Apple for personal use. The introduction of the Mac App Store, subsequent lackluster performance, and introduction of scary warnings about running unsigned code in MacOS tell most all of the story. This latest update is just another step in that direction, making running unsigned code that much more difficult. The obvious endgame is to raise the next generation of Mac users to only use the Mac App Store for software, effectively replicating the golden goose that the iOS App Store has proven to be.
I don't think I agree with your conclusion about their endgame being a replication of the iOS App Store golden goose, but I do see how it could come about. The reason I disagree is because changes like this oft remind me of the problems Facebook used to have (still do?) with stolen tokens and cookies that would come about from people pasting JavaScript into their browser's devtools console.
Some poor schmuck would find a viral comment that said something like "Mark Zuckerberg doesn't want you to know how to get Facebook Premium! Copy and paste this code into your browser, ignore Facebook's warnings, they don't want you to get it for free!" And badabing badaboom, stolen credentials get sent to some server.
I don't work at Apple and can only speculate on their motive, but since I advise my family to use macOS, it's my hope that this change would prevent them from reading something like "download this scary blob and then ctrl+click on it to open it for free Photoshop! Ignore Apple's warning, Adobe has paid them because they don't want you to get it for free!"
It does make it slightly more inconvenient for me, but I think chromedriver may be the only unsigned code that I run regularly.
They have been ratcheting down on freedom in OS X (and then MacOS) since 2011 or so when I stopped using Apple for personal use. The introduction of the Mac App Store, subsequent lackluster performance, and introduction of scary warnings about running unsigned code in MacOS tell most all of the story. This latest update is just another step in that direction, making running unsigned code that much more difficult. The obvious endgame is to raise the next generation of Mac users to only use the Mac App Store for software, effectively replicating the golden goose that the iOS App Store has proven to be.