Let's just look at it in context here. Star Wars additions will be nice and will boost DHS' attendance but we are talking about it taking over the flagship park of, well, the world basically when you factor in attendance.
2017 attendance the Magic Kingdom had 20,450,000 while DHS had 10,722,000 and both Epcot and Animal Kingdom are over 12,000,000.
How is this even possible that it will surpass the Magic Kingdom? Not even Disneyland or Tokyo Disneyland have surpassed it in this century in any given year.
1. You underestimate how popular Star Wars is, even today. The two lands will be the biggest and most popular "lands" ever built in Disney parks. It will boost DHS's attendance drastically, and reach capacity like no other WDW park has ever reached.
2. I agree that it won't ever surpass MK in attendance, but the gap with MK and it's three other WDW parks will become closer when it's all said and done, and SWGE will be a more popular land than any land in MK.