It's not like open source projects are immortal. Some die due to poor management, lost developer interest, lost funding, or some fork becoming more popular. Or all of the above.
Change is gonna come someday. Don't get left behind because of sentiment. This isn't a judgment on the Linux project, this is reality.
Mobile operating systems such as IOS have become huge and they're many new industries that look promising (my guess is IOT based one like alexa and Google home but who knows).
However, ios makes most of the mobile profit just as windows and Mac do over Linux for desktop (I have only used Linux in server industry so I don't know how they fair in those markets too much).
In that light, How about this prediction.
Linux will continue to dominate in the server market for a long time as businesses factor in the cost of os licenses and the supply of linux users is big.
Consumer markets like desktop, mobile, whatevers next will flock to the lower personal cost (through customer time investment in learning the os).
It's hard to imagine now, but it can happen.
Or it will be GNU Hurd.