Even if a third park were some generic collection of IP lands cloned from other parks, it’d still be better than a shopping mall. No matter how you spin it, it’s still more rides, more entertainment, more themed experiences, more events, and more to do at Disneyland than the two existing parks can ever offer. It spreads out crowds over three gates instead of two, and it all but guarantees non-locals will spend another day at Disneyland.
Also, the economics of a mall don’t even make sense to me. Who’s the target customer? Locals in OC have plenty of malls and dining/entertainment districts to choose from already. SoCal locals outside OC aren’t going to drive 2 hours to go to a mall that’s not meaningfully different from malls and shopping districts they have at home—including some of the country’s most popular—and they’re definitely not going to waste any part of an already packed Disneyland day or weekend trip there. You can maybe make a case that some people traveling from out of state or country might take an hour or two detour after visiting the parks before moving on to the next stop on their California trip, but there’s a lot more competition for quasi-free part-day activities in Anaheim/SoCal than on the outskirts of Orlando.