Keep in mind that for Disneyland specifically, it was the only park there for 45 years. Eventually, they would run out of serious room for expansion, even after alerting the RR many times (even in Walt's life) to make space.
The Magic Kingdom does not, and was built to not, have such constraints. Since 1980 however, the park has only had occasional changes to its footprint or real expansions (Splash, New Fantasyland and TRON). Significant replacements and changes in MK did not begin until the 90s, and a lot of that was fueled by the same business philosophy that at the same time closed all the unique stores. Sure the are reasons why something may be replaced vs real expansion, but for 45 years, Disney has been reluctant to use any space in its most popular park that wasn't previously occupied before. Cars and Villains continues this trend.
Remember, 20,000 Leagues sat idle for a decade before it was demolished for possible (not immediate) replacement