For professionals, I don't see mirrorless replacing single lens reflex anytime soon. Thats because of numerous factors, but the range of lenses and accessories is a key one.
Mirorrless' advantage is also its disadvantage. They can be small, which is great for your average shooter, perhaps street photographers and likely any pro that wants to carry a smaller camera sometimes. The negative is that they are too small, many pros will want something they can hold properly, especially if you are going to stick a big heavy lens on the end of it. There is also the fact that no matter what happens, I can't see people taking you seriously if you turned up to shoot a wedding or commercial studio work if all you had was something the size of a compact! In other words, big camera = pro!
Spending money on things in Photography really comes down to a couple of factors....
- If you are a Pro and know that by spending money on the kit it will earn you money, you need it.
- You are somewhere inbetween and take a lot of photos, and can easily afford to spend money on kit because its something you enjoy doing.
If you don't fall into either, then spending a lot of money on photography gear probably isn't worth it. As for mirrorless, it entirely depends on what and how you shoot as to whether its a worthwhile investment for yourself.
