You have no idea how many horse races and other top pro sports i have shot in four decades of my pro career. Certainly the photographer's skill makes more difference than any certain gear. I have shot TWO Triple Crown Winning Races as well as numerous other classic horse races, turf, grass, thoroughbred, quarterhorse, steeplechase, show horse etc. I have also shot top Pro Football, Stanley Cup finals, Four World Series, NBA Finals, NCCA Basketball Finals, Pro Soccer, Men and Women's Pro Golf, Pro Tennis including US Open, Olympic Athletes swimmers and Track and Field, NASCAR, Indy Car, CART, IMSA, SCCA, Hydroplane boats, Air Races, and more.
I have won World Press and POY Awards with my sports photos.
Look at the links to the mirrorless shots from the Winter Olympics, US Open tennis and US Open Golf. Fast action is no problem for mirrorless cameras that can track ,focus and shoot at 20fps ,with NO mirror blackout like DSLRs. Get real here. There is no longer A SINGLE thing that a DSLR can do that a mirrorless can't also do nowadays, and often the mirrorless can do it better,faster and for less money.
Was there ever a shot I got out-of-focus when shooting sports and racing in specific, sure. But that has happened with my top Canon and Nikon DSLRs and well as any mirrorless. The fact remains that the latest Mirroless systems in Sony's trilogy of top cameras the A9, A7RIII, and new A7III can capture any sport or fast moving subject, with great accurate tracking and continuous focus as well as ANY Canon or Nikon DSLR on the market, including $6500 1Ds and 5Ds , and for less money.
DSLRs aren't bad and mirrorless good. There is no perfect camera system, none from anyone. But as a pro that can afford ANY system I want, I choose to switch to mirrorless for a host of reasons. There is nothing I miss about the quality, performance or use of my DSLRs. I made award-winning shots and lots of money with my DSLRs, and I continue to make stunning art and professional shots for my clients and my personal shots with my mirrorless gear.
DSLRs wont completely disappear as long as people are willing to buy them. But more and more folks will embrace the future of mirrorless tech, here now for all of us.
Cheers