Failure will be the long, slow death of macOS.
First, content producers will abandon the Mac. Content producers have been the dominant part of the Mac's customer base for as long as the Mac has existed, but over the last few years Apple has been increasingly neglectful of that segment of their market. Letting the Mac Pro die on the vine, discontinuing Aperture, removing ports from the MacBook Pro, etc. At some point, the pro market will get sick of it and abandon ship, and this is twice as likely with Microsoft going out of their way to court creative professionals.
With the pros gone, there won't be a huge market for Macs anymore. There will be two types of people left: those who want a Unix machine with a user-friendly UX and iOS/watchOS/tvOS users who want a PC that will offer comprehensive integration with their gadgets. Those markets aren't huge.
The enthusiasts are a fickle market: they're just as likely to follow the pros in their exodus for most of the same reasons. On top of that, Windows 10 is becoming more and more attractive: the new Linux subsystem will scratch that "Unix box with friendly UI" itch just fine while also having all the compatibility benefits of running the most popular desktop operating system in the world (no more compromises for gamers!).
The consumers are going to find that the extra integration with iDevices isn't really a big deal, especially if the iDevices are their main machines. If you mostly use your iPad when you're at home, do you really need to keep buying new Macs? Why not just keep an old Mac around in case something comes up where you need to use a desktop and just not buy another? And if your old Mac does die, you can always just save some money and buy a cheap Windows PC... sure, you won't get Continuity, but did you ever really use it anyway?
So once the pros go, macOS and the Mac line will be on life support for a few more years, marketshare continuously shrinking until Apple stops updating the product line entirely, shifting to release only security updates, and then it'll stagnate a few more years before Apple stops Mac sales entirely. Just like the classic iPod.