Only a major problem for those who - for whatever reason - believe that there is some sort of major difference between a mirrorless camera and a DSLR. I have owned mirrorless cameras since about 2105. For a number of years a had both a mirrorless and a DSLR. I never really referred to them that way - they were just my 2 ILC cameras. The reason I liked the mirrorless was so that I could better see what exposure compensation I needed to make in the EVF, but for the most part, I took pictures with both cameras and usually it didn't really matter which one I was shooting with. Since 2015, EVFs have gotten better and they will no doubt continue to improve. It seems to me, that there will no good reason not to choose mirrorless over DSLR, and in fact, there are now numerous other reasons where mirrorless has the advantage.I would like to thank everyone who took the time to share their impressions and expectations. I see a common path regarding the future of the DSLR that even if we still can use a 1DXIII for the next 5, 10, or 15 years, it seems it will be it. If DSLR has a dead-end in the "near" future, we should also expect more lenses to be discontinued and fewer firmware updates. And that is a major problem, sadly so.
Mirrorless is just another way to make a camera. For whatever reason (it's what people tend to do) we have divided cameras into 2 categories as if they were distinct and different. They really aren't.
For those using their DSLRs for the next 10 years - you still have a much bigger selection of lenses to choose from and will have for many years to come. You can also get them much cheaper on the used market. I see no major problems anywhere for DSLR users who want to continue using their existing DSLRs.