Tim Cook says iOS will not merge with Mac OS
Apple CEO Tim Cook explained the reasons why Apple keeps iOS and Mac OS separate in an interview published in Thursday's Sydney Morning Herald. Cook said that both operating systems are "incredible," because Apple pushed them to do the things they do well. Merging the two would force Apple, in the CEO's words, "to make trade offs and compromises."
This all sounds contrary to Bloomberg's December 2017 report, which stated that Apple was working on a plan called "Marzipan" which reportedly involved combining the Mac OS and iOS codebases. But after Cook's denial, what we could see is Apple allowing iOS and Mac OS developers to share tools. This might give iOS developers the incentive to write apps for Mac OS. While this wouldn't be the same as iOS and Mac OS users downloading universal apps, it might help Apple get the Mac App Store going; the latter hasn't been nearly as successful as the iOS flavored App Store has been.