"Until...all major modern OSes come with a built in scriptable headless browser engine, which would be a game-changer."
Ah, yes. Internet Explorer, they called it. Built right into the OS. A game changer. Funny how the meaning of something (goodness/badness/motives/...) can change as circumstances change.
Internet Explorer ran on macOS back when it was relevant. It got discontinued after Safari was released. Also, Internet Explorer was ported to various Unices. It just never got ported to a Linux-kernel based OS.
If software like this is FOSS, relies on open standards, and is designed architect independent it can be run on virtually anything. Including something which is rising in popularity.
Not only was there IE for Mac -- at the time, it was actually a pretty decent browser. Bit rotted over time, but there was a stretch when your choices were a really janky Netscape and IE for Mac, and the choice as I recall it was pretty easy.
Not only was it pretty decent, it was far more standards compliant than the Windows IE. I don't think they actually shared much code wise. At least developing for IE on Mac felt like adding another browser to test.
The first computer I bought with my own money was an iMac running at 266 Mhz (not the first gen running 33Mhz slower!), Mac OS 8 something and Internet Explorer 5 I think.
Google didn't exist or wasn't evil yet and finding things was quite difficult specially at dial-up "speed".
Ah, yes. Internet Explorer, they called it. Built right into the OS. A game changer. Funny how the meaning of something (goodness/badness/motives/...) can change as circumstances change.