It used to be video professionals and graphic designers. Now it's developers and web designers. Developers don't need SD slots or a screaming GPU (although you can never go wrong with a better GPU). I worked off of a 2012 MBA with an external display for years, and it still fits the bill! Light enough to carry around between work, home, and coffee shops, powerful enough to run a local web server and all the apps I used for development and 2-3 screens, and enough battery life to get a day's worth of work done.
Apple will never drop MacOS or their Pro level computers because their Pro line is what makes their iOS App Store revenue. Their definition of "pro" is just going to be less video-centric, and more developer-centric.
Their recent slate of choices makes me wonder how they are being developer-centric. Or if they understand developers at all. They sure don't make a point of showing off developer-centric features, innovations, and upgrades.
They spend a week doing that every year at WWDC with thousands of people in attendance.
Even today, at a consumer-focused event, they made a point of telling people that developers could hook into the Touch Bar for their own applications.
They'll probably have an IDE on the iPad Pro, AKA the future of personal computers according to Cook, before we even see a hardware refresh with Kaby Lake.
Edit: Added "in startups"
Honestly I'd guess the main people keeping the Mac line afloat at Apple are probably their own engineering staff. Imagine having to develop Apple products on another OS, how frustrating that would be
That future seems to almost be here.
Or the ecosystem will be abandoned.