> What's the point in getting one over an iPad?

Better keyboard, larger screen, more connectors. And despite a lot of devs using macs, most of the userbase aren't devs, but are instead designers, artists, or just average people like college students etc. Most of those don't need third party stores, or execute any type of software outside of an apple approved list. Give them their specialist DAW, Adobe suite, etc. and they are content.

Irrelevant. If developers abandon a platform (no matter how small a percentage they are) then the platform dies. Especially one like macOS where the only true environment to develop native compatibility is one of their machines itself.