I don't know much about the current state of the art with mirrorless, but while a lot of things can eventually be compensated for (such as sensor size, PDAF capability (with hybrid AF chips, lens and strobe compatibility, etc), one thing that they can't do is give you an actual direct, through the lens image path.
A mirrorless can only ever show you what the sensor can see, never what is actually there. Anything that extends beyond the range of what the sensor sees won't be visible to you and that can have an impact on shot composition. Starting out, it might be nice to be able to see things how the sensor sees it to know exactly how it will come out, but once you have some experience, it is nice to be able to see what is actually there to know what you are and aren't capturing.
Battery life may also be another factor depending on the shooting environment. Mirrorless has to run the LCD, so it gets much less battery life than a DSLR can get when using the viewfinder, particularly if you get good at not having to spend a lot of time checking photos.
Room for dedicated buttons is also nice. It is just a convenience thing if you have time, but time you don't keep the customer waiting is time that the customer gets to keep and time you can spend working with another client, so it does have a benefit to both you and your customers.
There is also of course the "professional" look of a DSLR that shouldn't be underestimated when it comes to marketing yourself either. If you want to do commercial work, having people think your gear is big and fancy and expensive is almost if not more important than the actual quality and cost of your gear. This gap may also close, but having customers feel like they are getting their monies worth and that it isn't something they could have done with a point and shoot and a tripod is valuable for customer satisfaction.