No, I think people who have developed a UIKit app are going to port them to OS X because of Marzipan.
The incentive is easy port easy reach of a new, even if smaller, potential market.
To make the idea even clearer, if "porting iOS to OSX with Marzipan" was just a true/false toggle on XCode, almost everybody would toggle it and sell their iOS apps on the Mac App Store -- since it would take no time to do it, and it will reach a few dozens of millions on untapped potential customers (desktop mac users).
Well, it's the same situation, but now with Marzipan instead of trivially easy, like with the imaginary XCode toggle, porting an iOS app will be just "much easier" as opposed to "considerable effort" that it was before.
Also, I understood your comment "People with iOS apps on the App Store are a small subset of developers. That's the reality now." as meaning, "people with iOS on the MAC App Store are a small subset of developers" -- and responded to that.
If you meant: "People with iOS apps on the _iOS_ App Store are a small subset of developers" then that's crazy talk. They are hundreds of thousands of people making iOS apps -- and 1.5 or more million such apps.
You’re seriously underestimating the support overhead of having a Mac app. There’s way more hardware and software configurations than on iOS. The equation may not play out to make maintaining those Marzipan ports appealing for many.
The amount of UIKit developers is smaller than the amount of publishers. A lot of apps are made in bulk with web wrappers or template engines.
Let’s say there’s 200k UIKit developers in the world. That’s still clearly less than 10% of professional and hobbyist software developers.
>You’re seriously underestimating the support overhead of having a Mac app.
There doesn't have to be any "support overhead". You can simply offer the app as is, as tons of developers do. At worst, throw a "FAQ/common issues" page and leave it at that.
I have tons of apps, and very successful ones (judging from their publicity/store placement), that offer absolutely no support.
>Let’s say there’s 200k UIKit developers in the world. That’s still clearly less than 10% of professional and hobbyist software developers.
It's still clearly way more than needed to have a huge market. Heck, 5% of pro and hobbyists developers would still be enormously huge.