DSLR has much, much better battery life. The mirror also offers a degree of protection from dust for your shutter and sensor. There are less electronics that can break. These factors will guarantee DSLR usage by pros for the next 5-10 years at least.
Mirrors rarely (read: never) break through normal usage; the main advantage of mirrorless is not having to accommodate the mirror box which means cameras can be smaller.
There are tradeoffs that a DSLR makes to provide a through the lens optical viewfinder and phase detection autofocus. Mechanical complexity of a moving mirror and the bulk of a pentaprism in addition to the mirror bulk are among them. For some applications and some people the tradeoff is worth it for others it isn't.
If smaller size and lighter weight are more important then a mirrorless is more likely to be better.
The battery issue is not that simple. Mirrorless cameras need to keep the sensor active and the LCD or electronic viewfinder on during framing/composition, whereas the DSLR only needs to do this for the 1/60th of a second when the photo is being taken.
Bigger battery = The main weight of a DSLR body is already the battery. They can't go the Apple route of filling every possible space in the case with battery either because it needs to be replacacle.
If the reason for getting rid of the mirror is mechanical reliability I don't think it's a good idea to add something else that can break.
There isn't a technical reason, but rather which trade offs do you want to make? You can make a mirrorless camera that has every feature of an DSLR, but there is no point to do that if you lose the advantages of a mirrorless system (small and light).
I've had 2 DSLRs, and the first one broke a mount in the mirror in a way that it didn't flip up (automatically) any more. Took 50k images, but it did happen.
Mirrors rarely (read: never) break through normal usage; the main advantage of mirrorless is not having to accommodate the mirror box which means cameras can be smaller.