Building an app once for each platform is not economically viable for a lot of teams, 'serious' or otherwise.
Yes, but it's not Cocoa's problem.
It's Cocoa's problem because people stop using it for that reason. A toolkit no-one uses is a failure.
It's doing well - I currently have 2 Electron apps and dozens of Cocoa apps on my Mac. :)
We're talking about dev mindshare today. What you have installed is irrelevant.
I prefer native Mac apps to cross-platform ones. Serious companies should build native apps and keep UX guidelines of each platform in mind.