As long as people need a Mac for (at least essential parts of) iOS development, then Apple will keep selling Macs. Get really worried when Apple removes that requirement as the iPhone is their money press.

Being able to run the latest (or nearly the latest) macOS on 5+ year old hardware is a blessing and a curse. It's great when things are "fast enough" on old hardware, but when there are required processor functions that aren't available, etc., then it sucks to have to move to newer machines. And when you have to do that, it sucks to have to pay a premium for a 2+ year old design/CPU/RAM/storage interface.

It does seem that Apple is going to keep pushing the all-in-one iMac form factor. And it does make some sense - desktop PCs need a display, and by combining them with the processor/storage and using fairly high-quality screens, they can keep the price and their profit margins up. And make is more necessary for business customers to buy higher-end versions from the factory (we had to seal it all up on assembly because it was required by our elegant design!). And they can make faster, newer, UltaMegaBolt interfaces that let people attach fancy graphics cards, external storage, and all the rest, if they want to chase after such thing. And charge a fortune for them.

Apple went stupid with several designs in Jobs' day - The Cube is a notorious example. They do that. I expect them to keep making desktops for a long while, but not necessarily easily upgrade-able things like the Cheese Grater chassis. I don't expect an upgrade to the Mini (though I've got one and I think it's a great design) because they want to keep chasing the high end and there are too many cheap PCs that are that size.

You've got enough experience that a Hackintosh might be a worthwhile route for you when the time comes. Don't stay wedded too long to the old apps and OS - it's too easy to be left behind.

(One of my first job interviews after grad school was for a technical writer position. They wanted someone with Word Perfect 5.0 experience. I had 4.2. I got the job, but had to explain that they were basically the same.)

Good luck!