Disc brake fork with QR and vertical dropouts: worrisome?
As long as the skewer is set with sufficient clamping force this QR disc brake setup should work fine and from a practical standpoint there will be very little functional difference between this setup and a thru-axle.

If the QR is not sufficiently tight (i.e., you didn't set it with enough clamping force), you may notice a small shift in the axle position under very heavy braking. This isn't dangerous but annoying as the rotor will rub against the caliper until you reset the wheel position.  Thru-axles won't do this, but with some care it shouldn't happen in a QR setup either. Even though the axle can shift in some circumstances the axle will not come out as the "lawyer lips" around the outside edge of the drop out will retain the QR.  Even in the worst case where the QR somehow flips open while riding lawyer lips should still retain the wheel in the drop out.  This is why you have to unscrew the QR to get it out of the drop out.  By this point the wheel will be rubbing on the frame, not a great situation, but you won't be without a front wheel. 

Now suppose you open the QR, unscrew it a few turns, then go for a ride and heavily apply your front brake... then the wheel will come out of the drop out.  By then however you have made numerous critical errors in the setup.