I think a few years down the road, that there will be no discussion or competition between mirrorless and DSLRs. They are so close already in ability to take professional photos that I think people will barely differentiate between them. I own both a Canon 6D and an Olympus EM-1 and they both do an excellent job. I never refer or think about them as one DSLR and one mirrorless camera. They are cameras, that's all!
The competition will be between cameras and cell/smart phones. For the average person, their smartphone will do everything they need in terms of taking pictures. So camera sales will continue to decline and eventually plummet. When the current generation of kids reaches adulthood, smartphones will be so far advanced from where they are now, the vast majority will feel no need to use anything else. Older folks may continue to buy cameras - and, of course, professionals and serious amateurs, but the only arguments will be smartphone or camera? - not mirrorless or DSLR. That's what my crystal ball says!
There was an article I saw 3-4yrs ago. It said "DSLR will die within 5yrs"