To be honest, this will be a good thing for 95% of the Magic Kingdom attendees. This will not hurt your trip but help. The only people this hurts are those seduced by the fantasy of riding 7DMT multiple times in a row over and over with no wait.
1) The reason it sold out so fast was the capacity being that small. This is not even in the same ballpark as driving attendance like EMH drive attendance. I would bet ticket capacity was reverse engineered from Pinocchio. First they determined the number that they could serve in 1 hour, then they determined how many rides they needed to open along with 7DMT and Peter Pan. This is 1000 people or less who already had tickets to Magic Kingdom. This is not even close to EMH impact. I would not plan your day around this event.
2) This is only going to help pull down most wait times for 7DMT and Peter Pan. Do you really think that anyone who would pay for this event was not already a certainty to ride these rides anyway? Opening the ride an hour early = more people get to ride these popular rides per day (bonus capacity if you will). If you don't want to pay these prices, no worries. There are now 750-1000 people who no longer need to ride this ride. I doubt they used fast passes for them, nor will they do these rides at "rope drop." This won't hurt you. The park hours are not any different then what they were before, this is just extra time.
3) The only people this hurts minimally are those who booked a BOG breakfast for the sole purpose of getting a no wait ride at Peter Pan or 7DMT. Now you will have a short wait (but still ahead of all of the rope droppers). You still will be in good shape. If I had a BOG reservation but not tickets to the event, I would certainly not dump it as for the most part your benefit of beating rope droppers is in tact.
This event in among itself is actually a positive. The fear is the slippery slope this is treading on and the future application of this logic. For example, do they get rid of EMH? Does this discourage Disney from opening Norway early? Will they find additional hard ticket events to get more money per visitor? Will this be a way to limit attendance to Avatar? Star Wars? or Toy Story land?
People just need to relax and not let this minor change cause them to tear their plans up. If you had a good plan before, the plan is still good.