Exactly - it has to be holistic and actually planned out. If you want to run two airports in the same city, there really should be some form of quick connection between them, or each has to be so big as to be self-sufficient.

Moving an airport is even harder than just building one, because the airport often doesn't own all the businesses and land around said airport, and so is negotiation making those less valuable. And at some point, it's stuck - LAX is so enclosed in Los Angeles and LA is so big that you'd be quite far from it to add a new airport. You're more likely to repurpose Ontario or even a military base instead.