”I don’t recall seeing Atari specifically market the Mega ST to developers, but I suspect a lot of developers found the better keyboard and extra RAM to be worth the upgrade.”
There wasn’t such a thing as a general developer market.
When you didn’t have internet and cloud services and free Unix, how could you develop for something else than a specific platform and device?
If you bought a Mega ST to write programs, your target audience were still only the people who had a regular ST. You couldn’t reach anyone else. So the advantage was minimal.
The idea that there can be a developer market separate from the baseline end-user platform is quite new. It emerged around 2007-2010 when web apps became a realistic option and you didn’t have to be on Windows to target the 90% of people who are on Windows.