I know people who shoot weddings. That's all day work from breakfast till the end of the reception. They all use DSLRs with battery grips. They are already hauling in many extra batteries and charger arrays into the venue, an entire pelican case worth. For them to switch to mirrorless would mean 3 pelican cases to haul from where the bride and groom are getting ready, to the chapel, to a photoshoot location, then to the venue. That would mean a second vehicle for them. And lets consider the number of shots too vs the absolute time. A mirrorless camera in terms of shots might be good for 400 exposures at best. Shooting something like a wedding ceremony is a one take thing. You gotta get the shot, there is no asking the priest to back up again and repeat the vows, so your camera is shooting off like a machine gun and you hope one of those moments you have everyone in the frame with their eyes open without a funny look on anyones face. 400 exposures would go by in no time at all when you can take a twelve exposure burst in a few seconds.
There is a reason why when you see any big sporting event or anything with professional photographers these days, you still see a sea of DSLRs despite mirrorless cameras like the A7 having excellent sensors on the market for 10 years now.